I remember a long time ago on TOS that one of the posters (with a medical background - dang, I can't remember his name!) said that the tissue in the uh...anus....has something to do with the transmission of HIV. I'll have to see if I can find anything online. Basically the explanation was that because of the tissue structure and the way it allows viruses to enter into the body, that it was riskier.
(Oh my. I googled it. Go look for yourself.)