And, sorry, your question does deserve and answer.

The CDC's answer is:

"The reasons for these disparities are varied and not well understood."

reference


I am not an MD or biologist, but here is my answer constructed from what I have read and heard since living with one of the first diagnosed AIDs sufferers in 1981 (a hetero female nurse).

Several factors:
1) early on, gays got more attention than straights because they were reporting it more, and were more aware of it themselves.
2) the vagina is built for sex, the anus is not. It tears more easily, allowing infection to enter. Also, the vagina has beneficial flora whose purpose is to produce a pH and chemicals that are hostile to invaders.
3) Gay men do not have a vagina (see 2 above).