I am not sure the study is correct in its reasoning. In heterosexual sex the female has a very high risk of contracting AIDS if she partakes in an intercourse with an infected male. This risk is very very high and converges fast to 100% after just a few iterations. So maybe the chances per encounter are higher for a "receptive" homosexual but I suspect it doesn't explain why they are so much more prone to get infected.
I think there is a very simple explanation and it is that careless promiscuity in homosexuals is rampart. On the other hand among heterosexuals, the vast majority of females with AIDS in western society are whores. Other females are not nearly as active in terms of number of different sexual partners as typical homosexuals are. And on top of that even among very active females in the multiple partner sex scene they must be using allot more caution then typical homosexuals.
And one other factor is that in case of homosexual they obviously can and do switch roles and so each can both be at a high risk of contracting and at a high risk of infecting other with AIDS. That fact alone should make AIDS spread almost twice as fast in homosexuals then heterosexuals.