"An accidentally slipped out boob can hardly be equated to the idea of allowing a cheerleader to hump a horse" I highly doubt the boobage was "accidental"--that piercing really doesn't look fun or comfortable to wear. Those are the types of things women wear in order to be seen, not to be covered up under clothing.
Anyway, I suppect Kristinn was referring to the slippery slope argument and I don't feel like debating whether or not it is a valid argument to make--clearly it depends on the context, sometimes it is and sometimes it isn't.
Oh, but Valentine, it is. And since you decided to jump in, it was obviously valid enough of a debating point for you to insist I follow it.