Why are gay people at greater risk for aids than straight people?

Not so simple answer. Well.The HIV virus doesn't discriminate..But in general terms its the mechanical trauma of the anal sex and the easy transmisability. Condoms break way more easily in the rectum because of the tension of the constrictor muscles too.
3 reasons. (1) for partly historical reasons, the prevalence of HIV is higher among gay men, which makes the likelihood of exposure higher for each sexual contact. (2) rectal cells are more easily infected than vaginal cells. (3) unlike heterosexuals, gay men can be both insertive and receptive (top and bottom), which means they can be infected through receptive sex and then transmit through insertive sex.
Misinformed. Perhaps if you are asking only about the urban centers in america then the question would make sense. But worldwide your assumption falls apart. Hiv infections occur due to behavior that is risky. This includes intercourse, but also includes drug use with shared needles and other blood to blood contact. The epidemic in india and russia and in southern africa may open your eyes to the human tragedy.