... two men who are disease free and monogamous are not at risk.
The risk in sexual activities, be they hetero- or homo-, are with irresponsibility and unsafe practices. Neither of which are exclusive to either group.
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 sperm 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, [bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse] 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. sperm may have a similar impact on humans
Sources
-Gabriel Rotello,
Sexual Ecology: AIDS and the Destiny of Gay Men, p. 112, New York: Penguin Group, 1998 (quoting gay writer Michael Lynch).
-Jon M. Richards, J. Michael Bedford, and Steven S. Witkin, "Rectal Insemination Modifies Immune Responses in Rabbits,"
Science, 27(224): 390-392 (1984).
-S. S. Witkin and J. Sonnabend, "Immune Responses to Spermatozoa in Homosexual Men,"
Fertility and Sterility, 39(3): 337-342, pp. 340-341 (1983).
The claims above are not altered in anyway to fit any so-called agenda. Still, if you have doubt, research it yourself and provide concrete support for your claims.