You can get aids in two conditions. 1. if you have anal sex with a partner and then have vaginal sex. Kinda self evident. 2. If you have a partner with AIDS (say a girl and a guy and the guy has AIDS). Since the rectum isnt as "flexible" as the vagina the penetration can very easily break the lining inside of the rectum and the semen from the guy can flow into the blood stream for the lady carrying AIDs viruses. or similar case,is the GIRL IS HIV positive,and the guy gets tiny miniature ruptures on the inside of his penis because of the unlubricated rectum,and the ruptured rectum causes minute amounts of blood to seep into even the tiniest of ruptures on the man's penis, the blood carrying the HIV can cause AIDs on the man. THERE YOU GO.