I used to GM for a 50/50 male/female group... and I never got anywhere with the female players. I did get little "sumpin' sumpin'" from one of the gals in that group, but it was a good year after the group had broken up, and I wasn't RPing at the time. So, as a GM, I've never had to deal with this.
Anyone who bribes a GM for preferential treatment, be it food, booze, or nookie - keep on doing it.
HOWEVER, to all those GMs who consciously or unconsciously give preferential treatment to such players... shame on you. Of course, it's sometimes hard to do, and lord knows I've done it before - I'm a little harsher on my brother's character, than I am on, say, my Best Friend's character. I try to watch it, though.
I think if the "sex" player is getting only a slight preferential edge, well - that sucks, but what're you going to do? Now, if the GM *knows* he's giving preferential treatment to the "sex player" and keeps doing it.... I suggest chopping him up and putting him in the water supply somehow.