I thought of it, and here’s an interesting reason:

It used to be mindless hedonism. He spurned a rather drunken sorceress’ advances one night, because he absolutely, positively doesn’t have sex with drunken women (one might find this gallant, but it’s because they tend to vomit). Apparently, her mind was set on him, and so she cursed him: if he didn’t ejaculate at least once every day, he would feel horrible pain throughout his body. The pain would get worse every day. If he didn’t ejaculate for a fortnight, he would die. He hasn’t gotten past two days (jail processing days), though, and has only had to masturbate once. That isn’t to say that he still doesn’t enjoy it. He does. But now, he wants to find that sorceress, **** her again, and then kill her. Or maybe just kill her (then **** her?).

I have other ideas for him as well, but I’d like to see how they play out rather than “spoiling” them. They aren’t sex-related, either, and you asked specifically about his motivations there, I think. His motivations otherwise are pretty simple: survive. Really, he’s quite animalistic. But not so much that you’d catch him in a forest or something. Ugh. He’s not from a noble background, but he definitely fancies himself as a noble.

By the way, I usually don’t make characters completely based around sex. Ah, well.

And don’t worry, I won’t turn this into a porn. He might go off into the bushes by himself for a while, or go “information gathering” *cough cough*, but I’ll never be explicit. The minute there is an actual sex scene, by the way, I’ll bumume OOC that the person he’s with is trying to kill him, or something else interesting. :-P