Ok... in answer to the many many times people have asked why a guy would do it:
1. A guy's G spot is in the anus [for those that are inclined to take]
2. A chick's anus is a hell of a lot tighter than their pussy [for those inclined to give]
3. Having anal sex does not make you gay in the slightest. It is just another fun thing to do
4. A surprising amount of couples do it [and also there are people whose girlfriends 'go down' on them in that particular spot... you guys were worried about getting shit on your dick, imagine on your tongue]
Now for girls:
1. It is considered less conventional, more wild and kinky so it gives a sense of daring for a normal girl
2. For a lot of girls you can reach orgasm through the back door, not everybody obviously, but if you let yourself relax into it then I'm sure it would be easier
3. It can spice up a relationship, in the same way that doing anything different would
4. Some girls want to do something for their boyfriends... so they try anal
5. And then of course, there are the masochists who enjoy a bit of pain during sex
Having said all of that, I have never tried anal before, however I definitely will at some stage because the thought intrigues me. I figure if there is a chance it could be good for you, why not try it and find out? If it doesn't work, then you call it a failed experiment and be done with it.
I think a lot of girls are not comfortable enough with themselves... fair enough if the thought does not appeal to you, but don't tell other people they are 'disgusting' for having tried it, or wanting to.
[EDIT - if there are any more reasons that people can think of, then feel free to add to my lists]