Anal intercourse is the sine qua non of sex for many gay men.4 Yet human physiology makes it clear that the body was not designed to accommodate this activity. The rectum is significantly different from the vagina with regard to suitability for penetration by a penis. The vagina has natural lubricants and is supported by a network of muscles. It is composed of a mucus membrane with a multi-layer stratified squamous epithelium that allows it to endure friction without damage and to resist the immunological actions caused by semen and sperm. In comparison, the anus is a delicate mechanism of small muscles that comprise an “exit-only” passage. With repeated trauma, friction and stretching, the sphincter loses its tone and its ability to maintain a tight seal. Consequently, anal intercourse leads to leakage of fecal material that can easily become chronic.
The potential for injury is exacerbated by the fact that the intestine has only a single layer of cells separating it from highly vascular tissue, that is, blood. Therefore, any organisms that are introduced into the rectum have a much easier time establishing a foothold for infection than they would in a vagina. The single layer tissue cannot withstand the friction associated with penile penetration, resulting in traumas that expose both participants to blood, organisms in feces, and a mixing of bodily fluids.
Furthermore, ejaculate has components that are immunosuppressive. In the course of ordinary reproductive physiology, this allows the sperm to evade the immune defenses of the female. Rectal insemination of rabbits has shown that sperm impaired the immune defenses of the recipient.5 Semen may have a similar impact on humans.6
The end result is that the fragility of the anus and rectum, along with the immunosuppressive effect of ejaculate, make anal-genital intercourse a most efficient manner of transmitting HIV and other infections. The list of diseases found with extraordinary frequency among male homosexual practitioners as a result of anal intercourse is alarming:
Anal Cancer
Chlamydia trachomatis
Cryptosporidium
Giardia lamblia
Herpes simplex virus
Human immunodeficiency virus
Human papilloma virus
Isospora belli
Microsporidia
Gonorrhea
Viral hepatitis types B & C
Syphilis7