Quote Originally Posted by whowhatwhen View Post
Basically, anal sex can cause tiny tears in the lining of the rectum through which HIV could enter the body.
Well, that and other STDs as well.
They are also necessary for oral sex as well (and a similar barrier is needed when done on someone with a vagina.) Diseases are also spread via mouth to genital contact. (I mention this because I am shocked by how many people I have met over the years who did not know a condom or barrier is necessary for oral sex and didn't know there was a risk involved with it.)