Stiles doesn't actually mean to become Beacon Hills High's go-to guy for free condoms and sex advice. It just happens.

Derek's expecting the new guy. He's expecting another mostly-useless college kid who doesn't know one end of a horse from the other, and Derek doesn't have the time to babysit. What he's not expecting is Stiles Stilinski, who carries on conversations with the livestock and gets maybe a little too chummy with Isaac and is really kind of stupidly perfect in every way. And it's just distracting, that's all.

Stiles didn't choose to give his life to the unicorns, not really; they were the ones who chose him. He didn't understand, then, what it was they were taking from him.

The whole thing starts with Stiles' naked body, which is, apparently, "perfect."