According to the CDC:

“Anal sex is the riskiest type of sex for getting or transmitting HIV.”

Causes “immunosuppression…”
…the immunosuppression in homosexual males that is reported even in apparently healthy or HTLV-III-antibody-negative groups may arise from seminal PGE2 received during anal intercourse.
“Prostaglandin E2 administered via anus causes immunosuppression in male but not female rats: a possible pathogenesis of acquired immune deficiency syndrome in homosexual males.”
S Kuno, et al.
Proc Natl Acad Sci U S A. 1986 Apr; 83(8): 2682–2683.

Gay sex is always getting kinkier and more violent:
There is a rising trend in high risk sexual behavior among men who have sex with men (MSM), with concomitant use of recreational drugs. Activities include fisting and unprotected anal intercourse with a partner who is HIV serodiscordant or of unknown status.
“Sexual trauma associated with fisting and recreational drugs.”
Cohen CE, Giles A, Nelson M.
Sex Transm Infect. 2004 Dec;80(6):469-70.

Gay marriage is no safeguard against HIV:
Over 50% of HIV transmissions in US gay men are from main sexual partners.
“Estimating the proportion of HIV transmissions from main sex partners among men who have sex with men in five US cities.”
Sullivan PS et al.
AIDS 23 (online edition), 2009.

Besides HIV, gay sex causes homosexual men to be at an increased susceptibility for a variety of sexually transmitted diseases:
Compared with other sexually active adults, men who have sex with men (MSM) are more frequently infected with several pathogens including cytomegalovirus, hepatitis B virus, and Kaposi sarcoma-associated herpesvirus…MSM in a San Francisco population-based cohort were interviewed regarding use of saliva by the insertive partner as a lubricant in various anal sexual practices. Among 283 MSM, 87% used saliva as a lubricant in insertive or receptive penile-anal intercourse or fingering/fisting at some point during their lifetime; 31%-47% did so, depending upon the act, in the prior 6 months.
“Use of Saliva as a Lubricant in Anal Sexual Practices Among Homosexual Men”
Lisa M Butler, et al.
JAIDS Journal of Acquired Immune Deficiency Syndromes 50(2):162-7 • February 2009

The majority of gay men engage in anal sex; and there is no right way to do it:
In the United States alone, receptive anal intercourse is practiced in up to 90 percent of gay and other men who have sex with men, according to International Rectal Microbicides Advocates…In one study involving nearly 900 men and women in Baltimore and Los Angeles, the researchers found that those who used lubricants were three times more likely to have rectal sexually transmitted infections (STIs). Another study that subjected popular over-the-counter and mail-order lubricants to rigorous laboratory tests discovered that many of the products were toxic to cells and rectal tissue.
“Use of lubricants with anal sex could increase risk of HIV.”
Microbicides 2010 (International Conference on Microbicides)
May 25, 2010

If you are a heterosexual male, your likelihood of contracting HIV is relatively small:
Over the 6 years there were 33,681 HIV tests performed on men, of which 17,958 tests were for heterosexual men. From these heterosexual men, nine tested positive for the first time…These nine cases included six men who had had sex with a female partner from the following countries: Thailand, Cambodia, China, East Timor, Botswana and South Africa. Two men had injected drugs and one had a HIV-positive female partner.
“HIV is rare among low-risk heterosexual men and significant potential savings could occur through phone results.”
Bush MR et al.
Sex Health. 2010 Dec;7(4):495-7

Young gay men are more likely to contract HIV from their older male partners:
…the Centers for Disease Control and Prevention has shown a resurgence of the HIV epidemic particularly in young MSM…MSM participants with recent HIV infection reported having sex with older partners.
“A Major HIV Risk Factor for Young Men Who Have Sex With Men Is Sex With Older Partners”
Brian J. Coburn, PhD and Sally Blower, PhD
J Acquir Immune Defic Syndr. 2010;54:113–114

A majority of gay men do not regularly practice “safe-sex:”
Among men whose most recent sexual encounter was with a main partner, 83% reported having anal sex, and 49% reported having anal sex without a condom during that encounter. Among men whose most recent sexual encounter was with a casual partner, 68% reported having anal sex, and 24% reported having anal sex without a condom during that encounter.
“HIV Risk, Prevention, and Testing Behaviors National HIV Behavioral Surveillance System Men Who Have Sex with Men”
20 U.S. Cities, 2011

Young gay men are more likely than heterosexuals to engage in sex with an older partner:
Over one-half (52.0%) of MSM aged 18–24 reported a recent male anal sex partner who was >5 years older…By contrast, only 7.9% of heterosexual men and 10.0% of heterosexual women in this age group reported a recent partner who was >5 years older.
“A comparison of sexual behavior patterns among men who have sex with men and heterosexual men and women”
Sara Nelson Glick, et al.
J Acquir Immune Defic Syndr. 2012 May 1; 60(1): 83–90.

Sexually transmitted diseases among gay men are becoming antibiotic resistant:
Among 79 cases of Shigella sonnei, 56 occurred in HIV-infected MSM, while 23 were observed in HIV-negative MSM. High resistance rates (>90 %) were found for doxycycline, tetracycline, aminoglycosides, all cephalosporins of first and second generations tested, and trimethoprim/sulfamethoxazole. In total, 54 % of cases were resistant to ciprofloxacin. Compared to negative subjects, HIV-infected MSM had a significantly higher rate of quinolone resistance.
“High rates of quinolone-resistant strains of Shigella sonnei in HIV-infected MSM”
C. Hoffmann, et al.
Infection. October 2013, Volume 41, Issue 5, pp 999–1003

More gay men are having unprotected anal intercourse:
Unprotected anal sex at least once in the past 12 months increased from 48% in 2005 to 57% in 2011.
Centers for Disease Control and Prevention
Morbidity and Mortality Weekly Report (MMWR)
HIV Testing and Risk Behaviors Among Gay, Bisexual, and Other Men Who Have Sex with Men — United States
November 29, 2013 / 62(47);958-962

A majority of gay men report more than two recent sex partners:
In the last ninety days, 32.7% men reported no sexual partners, 25.8% reported one-to-two sexual partners, and 41.5% reported more than two sexual partners. Most men reported meeting their sexual partners both online and offline. Most MSM (69.4%) reported less than two unprotected anal sex male partners in the last 90 days, while 73.1% reported protected anal sex with one or more men. The products most commonly used for douching – water, soap, and saline — are those, which laboratory and clinical studies have identified as damaging the epithelium. Thus, douching appears a candidate behavior that may plausibly influence both HIV and other STI transmission among MSM.
“Enema Use among Men who have Sex with Men: A behavioral epidemiologic study with implications for HIV/STI prevention”
Syed W Noor and Simon Rosser
Arch Sex Behav. 2014 May; 43(4): 755–769.

Gonorrhea and chlamydia are becoming antibiotic resistant in gay men:
Extragenital GC/CT was common among MSM attending STD clinics…Of MSM tested, 11.1% tested positive for urogenital GC, 7.9% for pharyngeal GC, 10.2% for rectal GC, 8.4% for urogenital CT, 2.9% for pharyngeal CT, and 14.1% for rectal CT.
“Extragenital gonorrhea and chlamydia testing and infection among men who have sex with men–STD Surveillance Network, United States, 2010-2012.”
Patton ME, et al.
Clin Infect Dis. 2014 Jun;58(11):1564-70.

Younger gay men are likely to have an older first-time lover:
The median age was 19 years. The median age at first insertive or receptive anal intercourse was 17 years. Half of men reported sex with mainly older men: these men were more likely to engage in receptive anal intercourse (48% vs. 25%) than other men. Most men had engaged in insertive (87%) and receptive (85%) anal intercourse in the prior 12 months with 60% and 53% reporting inconsistent condom use with insertive and receptive anal intercourse partners, respectively.
“Sexual behaviors and risk for sexually transmitted infections among teenage men who have sex with men.”
Zou H, et al.
J Adolesc Health. 2014 Aug;55(2):247-53.

Condom use does not guarantee safety:
Among Ontario MSM in 2009, an estimated 92,963 HIV-negative men had 1,184,343 episodes of anal sex with a condom and 117,133 anal sex acts without a condom with an HIV-positive partner. Of the 693 new HIV infections, 51% were through anal sex with a condom, 33% anal sex without a condom and 16% oral sex.
“HIV Transmission among Men Who Have Sex with Men due to Condom Failure”
Robert S. Remis, et al.
Published: September 11, 2014

Oral sex isn’t a “safer” alternative:
…oral sex has an important role in sustaining gonorrhea in a population of MSM by providing a pool of untreated asymptomatic infection.
“Oral and Anal Sex Are Key to Sustaining Gonorrhoea at Endemic Levels in MSM Populations: A Mathematical Model”
B Hui, et al.
Sex Transm Infect 91 (5), 365-369. 2015 Jan 16.

Although the risks of receptive anal sex are well known, gay men continue to engage in such behaviors:
70% of MSM reported receptive anal intercourse at least once in the past 3 months…
“Consistency of Condom Use During Receptive Anal Intercourse Among Women and Men Who Have Sex With Men: Findings From the Safe in the City Behavioral Study.”
DʼAnna LH, et al.
Sex Transm Dis. 2015 Jul;42(7):393-9.

Difficult to treat chronic diarrhea has been reported in gay men:
Shigella flexneri is an emerging pathogen in men who have sex with men; recent outbreaks related to sexual practices have been noted in this population in the UK and other developed countries. While majority of cases of Shigellosis present with gastroenteritis, some vulnerable patients with underlying immunosuppression can get complications like bacteraemia and may present atypically as an acute surgical emergency.
“Invasive shigellosis in MSM.”
Serafino Wani RL, et al.
Int J STD AIDS. 2015 Oct 1.

Anal sex causes incontinence:
The findings support the assessment of anal intercourse as a factor contributing to fecal incontinence in adults, especially among men.”
“Anal Intercourse and Fecal Incontinence: Evidence from the 2009-2010 National Health and Nutrition Examination Survey.”
Markland AD, et al.
Am J Gastroenterol. 2016 Jan 12.

HPV, a sexually transmitted disease typically found in women, is epidemic in the gay male community:
Anal HPV was highly prevalent in MSM (HIV positive, 88% and HIV negative, 78%).
“A prospective study of anal cancer screening in HIV positive and negative men who have sex with men; results of Analogy.”
Schofield AM, et al.
AIDS. 2016 Feb 1.

A majority of HIV+ gay men have experienced some form of anal trauma:
75% of the men had abnormal anal cytological/histological results. 41% presented with low-grade, 24% with high-grade anal dysplasia…
“The male ScreenING Study: prevalence of HPV-related genital and anal lesions in an urban cohort of HIV-positive men in Germany.”
Fuchs W, et al.
J Eur Acad Dermatol Venereol. 2016 Feb 1.

35% of gay men have a sexually transmitted infection:
One third (35%) were positive for STI. STI prevalence was significantly associated with using sex slings, felching, group sex, fisting, anonymous sex, and sex toys. HIV prevalence was 17% and was significantly associated with fisting, felching, enemas, and group sex.
“Beyond Anal Sex: Sexual Practices of Men Who Have Sex With Men and Associations With HIV and Other Sexually Transmitted Infections.”
Rice CE, et al.
J Sex Med. 2016 Feb 4.

Gay men have increased prevalence of antibiotic resistant gonorrhea:
A total of 5,093 isolates were collected in 2014. Of these, 25.3% were resistant to tetracycline, 19.2% to ciprofloxacin, and 16.2% to penicillin (plasmid-based, chromosomal, or both)… The percentage of isolates resistant to tetracycline, ciprofloxacin, penicillin, or all three antimicrobials, was greater in isolates from MSM than from MSW.
“Neisseria gonorrhoeae Antimicrobial Susceptibility Surveillance – The Gonococcal Isolate Surveillance Project, 27 Sites, United States, 2014.”
Kirkcaldy RD, et al.
MMWR Surveill Summ. 2016 Jul 15;65(7):1-19.

Younger gay men are less likely to practice “safe-sex;” but are more likely to use drugs:
When compared to MSM 50 years or older (HIV prevalence 2.5%), significantly higher rates of unprotected anal intercourse, bacterial STI and stimulant substance use were reported among young MSM, whereas total number of partners during the last 12 months did not differ. While higher rates of stimulant substance use may explain the higher rates of condomless anal sex among young MSM, the observation of a recent study that condom use errors and problems, such as breakage and slippage were very common among young MSM…
“HIV Infection Rates and Risk Behavior among Young Men undergoing community-based Testing in San Diego”
Martin Hoenigl et al.
Sci Rep. 2016; 6: 25927.

In the US, 60% of syphilis cases were in 2% of the general population – gay men:
In men, sores can occur on or around the penis, around the anus or in the rectum, or in or around the mouth. These sores can be painless, so it is possible to have them and not notice them. Correct use of condoms can reduce the risk of syphilis if the condom covers the sores. However, sometimes sores occur in areas not covered by a condom. It is still possible to get syphilis from contact with these sores
Syphilis & MSM (Men Who Have Sex With Men) – CDC Fact Sheet (January 2017)

Source: HIV Infection Risk, Prevention, and Testing Behaviors among Men Who Have Sex with Men
National HIV Behavioral Surveillance 20 U.S. Cities, 2014

Pictured above: Max Oppenheimer, The Bleeding Man (1911).