This could be bullshit, but I've read that a primary determining factor in whether a serious relationship will last very long or not is if the two people involved are relatively equal in terms of how attractive they are to members of the opposite sex (or, members of the same sex I guess, for gay relationships). If there is too much disparity, one starts feeling shortchanged, the other may feel inadequate or jealous. For super-attractive people, they may have a hard time finding people of their caliber.
I dated this former model briefly, and she complained about the fact that most of the guys she went for were total players. She was gorgeous, though, and most of the guys she went for were also gorgeous and could have been models themselves. I pointed out that, if a guy is so attractive that he can have pretty much any girl he wants, there's going to be little hope of him wanting to stick with just one. Not to say that's an impossibility, but to do it would be going against a very strong biological predisposition, which in most men is overrided by social constraint and the need to conform to find partners. If you're the type of guy who has models hitting on you all the time though, that pressue to conform in order to find partners is gone. If you piss one off, there's another super gorgeous girl waiting in line. Same is true for rock star/celebrity/super rich men. So for the girls who are so attractive that they can't be happy with anyone less than the guys that every woman goes for, then yes, I can see the women being unfulfilled or turned off from sex as a result.
The way you phrased the subject heading of this thread, I thought it was going to be about attractive girls being bad in bed, and I was going to comment that I've noticed the less attractive girls I've been with have often been the freakier ones or the ones who were more eager to please... but having sex with someone you find to be extremely attractive in itself makes the sex so much better: you're more aroused, your body reacts more to everything that's going on; so I think that more than makes up for it.
As for your closing hypothesis... I would have to heartily disagree. I know lots of extremely attractive extremely promiscuous women. I also know a few fairly homely virgins. What you said might be true in a couple isolated cases, but I doubt there is a strong correlation. Furthermore, especially for guys, the less attractive you are the harder it is going to be to find sex so it doesn't really matter so much if you want to have it more. Women can get laid pretty much whenever they want to, regardless of how they look and without having to pay for it, so I don't think the phenomenon you described is going to affect much in their minds. If a gorgeous woman is bad in bed I would assume it has more to do with her not feeling so much like she has to please her partner.
How a girl fucks, as far as I have seen, does not depend on whether she is beautiful or not. Depends on all manner of things, experience, natural prowess, mentality to a certain extent. The guy as well, if the guy just doesn't feel right why should she bother?
