The Monkey's story about the tapeworm is BS. A long tapeworm attaches itself in the intestine closer to the stomach with it's head end towards the stomach and it's behind closest to the, ehr, behind. Without attaching itself, it would get pooped out. Also, they are hermaphrodites with male segments at the top and female segments at the bottom. They release sperm from the top segments which travel down the intestine and impregnate the lower female segments. If the worm was the other way round, it wouldn't work. Also, one which has been pulled out with it's head first would not be able to crawl back into the ass and finally, I just don't believe in dick biting tapeworms! Good story, though.
And I have to say, for a sexual experience thread, isn't it getting a bit close-minded?
edit :
Parasites can of course be a concern with sex. If you don't wanna catch other people's parasites, be careful with anilingus.