Anal sex, practiced properly, is as safe as any other kind of sex. And people do it because it feels good--the anus can be an intensely erogenous zone. In fact, far more straight people than gay people practice anal sex! The anus contains more nerve endings than any other part of the male body, and more than any part of the female body except the clitoris. It's no wonder that anal sex is a part of many peoples' sex lives.
"Anal sex" can range from simply stroking your or your partner's anus with a lubricated finger, to actually sliding some fingers inside your partner and stroking them, to full anal intercourse.
All these things are physically very pleasurable, and if you simply wash your butt, there's nothing repulsive about them. The anal taboo is very old, but there is no necessary medical reason for it if you know what you're doing. If you're concerned about staying clean, by all means make sure you've gone to the bathroom before playing, and wash your ass--outside and, if you wish, inside, with an enema. If you want to feel clean in order to enjoy anal sex, it's not hard to be as clean as you want. (It is also very important, though, to use safe sex techniques, which I describe a bit further on.)
The main guidelines for anal sex are Communication, Relaxation, Lubrication. You see, your anus consists of two rings of muscle, dubbed the external and internal sphincters. Your external sphincter is under your voluntary control--you can relax it at will. But your internal sphincter is not under voluntary control. If you are tense, your internal sphincter will be tight, and trying to force anything into it will hurt, which will make you (and it) even more tense. So the rule in anal sex is to go slowly; you can't force your way into enjoying it.
Talk about what you're going to do before you do it! Don't just roll your partner over and surprise them; they won't be relaxed and it won't be fun. Make sure you both are comfortable with the idea of anal play. Relaxation: listen to your body. If your ass wants to be played with, you will know; if it doesn't, don't rush anything.
Your anus doesn't lubricate, so you need to use a WATER-SOLUBLE lubricant such as KY Jelly or Probe. Use LOTS of it; it's clean! The more lube you use, the more comfortable you will be. And finally, communication again: if you haven't played with your anus before, the sensations will be intense and strange. You may feel like you are having a bowel movement when your partner slides their fingers out of you; it takes some experience to realize that this feeling is deceptive and that what you're feeling won't result in a soiled bed sheet.
It's not enough to just clean your anus, though; your partner should also use a latex barrier (a glove for fingering, a dental dam or a piece of (non-microwave-able) Saran Wrap for licking, and a condom for fucking) when having sex with you. This is true in general, but especially true for anal sex; unprotected anal sex is the riskiest kind of sex with regard to transmitting STDs of any sort. Also, using protection often increases the sensation of safety and cleanliness, which helps many people relax and enjoy the experience more. (Some say that anal play isn't as risky as all that. The facts are that in some cities intestinal parasites, spread by unprotected anal sex, have been considered a serious sexually transmitted public health problem, with thousands of people infected. Decide for yourself how much risk you want to accept.) And anything that has come in contact with the anus should be cleaned thoroughly (or thrown away, in the case of latex barriers) before coming into contact with the mouth or vagina.
I already mentioned that it's not a good idea to force anything. Let me be more emphatic: if you feel pain in your ass while you're having anal sex, STOP. Too-rough anal sex can stress and possibly tear the anal lining, which can lead to very serious infections. Anal sex does NOT mix with force, and should NEVER be used as a way to inflict pain. And if you find yourself bleeding from the rectum, go see a doctor IMMEDIATELY. (Don't be embarrassed--they've seen it all before... just get yourself taken care of!)
That said, I need to clarify what I meant by STOP if you feel pain. That is what you should do: stop moving. The pain may just be your sphincter muscle complaining about stretching a bit, and when you stop pushing it will stop hurting--and possibly relax some more. If it doesn't stop hurting when you stop moving, THEN you want to pull out (slowly) and take appropriate action. If it does stop, wait a little, then begin again... your ass will let you know if it wants to stop altogether. (So pay attention to it! Getting drunk is NOT a good idea, as you don't want to block out any pain you may feel. The FAQ List No-Prize for Worst Sexual Product goes to an "anal lube" that contained oil (and therefore couldn't be used with gloves or condoms), AND which advertised itself as being best for anal sex BECAUSE it contained benzocaine "for greater comfort"! If anyone did hurt them- selves through using it, I hope they sued the hell out of the company.)
If you want more information about anal anything, see Jack Morin's book
"Anal Pleasure and Health", by Jack Morin, Down There Press, Burlingame, CA. This book has the complete lowdown on all aspects of anal sex-- safety, hygiene, emotional issues, you name it. Extremely valuable information, well presented.