It is harder for women to spread the disease, but not impossible, which is why you see such high infection rates in the developing world.
The fact is that if you have multiple sexual partners, gay or straight, vaginal or anal, male or female, you are at increased risk of HIV and STD infections. There are all kinds of other linked factors, as there always is (such as circumcised men being at slightly less risk), or, as you point out, women being less likely to spread the disease because of the structure of the vagina versus the rectum. But as evidenced by HPV, herpes and hepatitis infections, vaginal intercourse doesn't confer some magic immunity against viral STDs. All it takes is an exchange of bodily fluids, so if the man has even a small, possible even invisible lesion on the skin of his penis, he's at elevated risk of infection, and when the person he is having sex with is at later stages of the infection, and viral counts in bodily fluids like blood or vaginal fluids, those risks get higher.