Sexual activity is one of the primary ways AIDS is transmitted. AIDS is transmitted particularly by the transmission of blood or semen of an infected person into contact with the blood of an uninfected person. Sex involving penetration of the penis into either the vagina of a woman or the rectum of either a woman or a man has a very high risk of transmitting the disease. It is felt to be about four times MORE likely for an infected male to transmit AIDS to an uninfected woman in the course of vaginal sex than it is likely for an infected woman to transmit AIDS to an uninfected male. This probably relates to the greater area of moist tissue in a woman's vagina, and to the relative likelihood of microscopic tears to occur in that tissue during sex.
Oral sex is an extremely common form of sexual activity among both gay and straight folks.
AIDS virus and infected cells most certainly cannot survive the acid environment of the stomach. Yet, it is still felt that there is a chance of catching the disease by having oral sex with an infected person. The chance is probably a lot smaller than in the case of vaginal or rectal sex, but is still felt to be significant. AIDS is also transmitted among intravenous drug users by the sharing of needles. Testing of donated blood for AIDS has massively reduced the chance of catching AIDS from blood transfusions.