Anal sex is a form of human sexual behavior which encompasses many sexual acts involving the anus anal cavity sphincter valve and/or rectum. The term anal sex however is often restricted to anal intercourse: the insertion of the erect penis into the rectum.

It is a form of sexual behavior considered to be comparatively high in risk due to the vulnerability of the tissues and the septic nature of the anus. As the rectal mucosa provides little natural lubrication a lubricant (such as a personal lubricant for example) is most often required or preferred when penetrating the anus.

In modern times particularly in Western cultures anal sex has been popularly associated with gay and bisexual men.

Among gay men who have anal sex some consistently take the top (insertive) or bottom (receptive) role but this is not always the case: some men who have anal sex act as both top and bottom at different times. This is known as "versatile" or "switch". Among gay men who have anal sex some consistently take the top (insertive) or bottom (receptive) role but this is not always the case: some men who have anal sex act as both top and bottom at different times. This is known as "versatile" or "switch".

In men anal sex can produce a pleasurable sensation due to the stimulation that the partners erect penis causes when rubbing or massaging the prostate (see below). Penetration can be painful for some men depending on their rectal anatomy or on the amount of lubrication used during penetration. While some men obtain strong sexual pleasure from being penetrated occasionally becoming so aroused as to even avoid the need for penile stimulation to reach orgasm others may find it discomforting at first or at the other end of the spectrum completely painful or unenjoyable.

In modern Western countries anal sex is reported as being a widely practiced if still minority form of intercourse between men and women. In some cultures receptive anal intercourse by heterosexual partners is widely accepted

One reason is that there is very low risk of unwanted pregnancy via unprotected anal intercourse (though this is not an absolute guarantee because semen can leak from the anus cross the perineum and enter the vagina).

Also anal sex is sometimes seen as preserving female virginity because it leaves the hymen intact. Another reason is that the anus is considered to yield more tactile pleasure for the penis being tighter than the vagina.

Alternatively the male may take the receptive role in heterosexual anal sex usually aided by sex toys. The practice of pegging in which a woman wearing a strap-on dildo penetrates her male partner is gaining acceptance. Moreover due to the proximity of the prostate gland to the rectum it has been suggested that males may derive greater satisfaction in this manner than females