Here's a video of a Colorado woman testifying on the civil unions bill in that state, arguing that anal sex isn't something we're supposed to do because we have anal sphincters. I'm guessing some people couldn't keep from giggling during her testimony, but she's really getting at a lot of people's discomfort with male homosexuality. Usually they don't explain it in these terms, but for a lot of people this is what it comes down to. (at 0:53)
While there are definitely arguments that can be made about the "purpose" of a body part, it's always left open to debate. She picks on the anal sphincter, which, in my humble opinion, is there to keep poop from falling out by accident. And the hymen exists too, but no one argues that it means "keep out" of the vagina.
But who knows, maybe the sphincter is there to stop buttsex. I wonder how one would explain why anal sex is physically pleasurable, assuming she's correct? Why are there so many nerve endings there that give sexual pleasure when stimulated and, in men, a prostate on the inside that can be reached by a human penis?
To someone who's new to humanity, all that would be taken as a huge, neon, blinking sign left by God/Goddess/Gods/Nature/the Flying Spaghetti Monster pointing us towards anal sex.