From Encyclopedia of Sex and Sexuality
Anal intercourse takes place when the penis is inserted through the anus into the rectum of a partner. While many heterosexual couples try it occasionally, it appears to be more frequent among homosexual males. Regardless of a partner’s sex, the penetration may be painful unless the recipient is very relaxed and a water-based lubricant is used (not Vaseline). Also, because the anus is normally quite small and not designed for heavy thrusting, injury may result from tearing of the anal sphincter or surrounding tissue. While engaging in anal intercourse, the penis, when removed, should not be placed in the partner’s vagina or mouth before being thoroughly washed because of the possibility of carrying infectious bacteria from the anus to other parts of the body.
AIDS researchers believe that anal sex is the single most dangerous practice in terms of spreading HIV, the virus that causes AIDS (especially for the recipient). Therefore, unless the partners are absolutely sure that they are both free of HIV, they should always use a new latex condom with generous water-based lubrication.