| We're so glad you came|
|Reach around the subject|
“”There was a bartender named Anal Johnson… He always kept things nice and clean.
|—Ballad of Rock Ridge from Blazing Saddles|
Anal sex is the act of stimulation or penetration of the anus for sexual pleasure. This is mostly accomplished through the stimulation of the back part of the prostate or clitoris. Some people like it, some don't.  We'd recommend going to sex education for better references on the subject, though.
For now, we'll just remind you: Things can get torn up in there. Lube up, communicate clearly, stop if it hurts — and stop if your partner asks you to — and don't ever stick anything that just came out of a butthole in a mouth or a vagina. While ass to mouth is probably safe provided that the ass and the mouth belong to the same person, ass to vagina can cause infections. And don't believe everything you see in porn. The performers generally clean their dicks between takes.
Anal sex and homosexuality
Some heterosexuals practice anal sex. Some women who have sex with women, too. And some men who have sex with men. However, many homophobes seem fixated on the idea that anal sex is the big thing about being gay.
Anal sex is common
in countries where a girl's physical virginity (hymen) is highly prized everywhere where people have sex. As such unmarried couples sometimes practice it in Muslim and Middle Eastern countries, notably Turkey. because it feels good.
In the UK, buggery (yes, that's an actual legal term) is still technically illegal between a man and a woman although no one is likely to be prosecuted for it. Comedian Stewart Lee actually ran a spoof campaign to legalise it.
In the US, anal sex was considered illegal in some jurisdictions until the 2003 United States Supreme Court ruling in Lawrence v. Texas effectively found that consensual anal sex in private was a right.
Anal sex and HIV/AIDS
If one partner is HIV-positive, unprotected anal sex carries a high risk of seroconversion for the HIV-negative parnter. The anus is a very thin piece of tissue (only a few cells wide) and the virus can be transmitted through small "microtears" in the anus of the "receptive" partner or the penis of the "active" partner. There is less natural lubrication in the anus than the vagina, making it easier for condoms to break during sex. Always use a condom during anal sex with a new partner, check frequently during intercourse to make sure the condom hasn't broken, and don't have unprotected anal sex with someone who isn't vigilant about getting tested for HIV and doesn't know their status. And don't "double-bag" it (use two condoms simultaneously); that actually increases the risk of condom breakage.