I believe it's illegal, there's precedent. It's considered a form of prostitution. A girl did it a while back, and ended up having to change it to "bidding on a dinner date" with her, sex optional. But it was one of those wink wink, nod nod type auctions. Like the guy who offered free Super Bowl tickets to a hot chick just for "company" prior to the game.
BUT...If GFY says it's cool, most judges will roll with that. 