There is a medical theory that most cancers and age related disease like dementia are caused by viruses. I would imagine gay men by virtue of their sexual choices and much higher IV drug use are exposed to hundreds of more viruses than a straight person.
Especially the bug chasers among them.
Not to mention parasites, bacteria, amoeba, toxins, (possible sperm in the blood-—one of the original(but incorrect) ideas for cause of acquired immune deficiency syndrome(aids)...