If she was ultra hot and was really up for it (and I didn't force it on her what-so-ever, I just happened to be 'there'), yes I would probably be weak willed and go with her (esp. if I was a bit tipsy anyway). I would still be nice and ask if its really what she wanted (pointing out that the timing was pretty inappropiate), and if she still wanted it, well yeah... but thats not really taking advantage...
The actual 'bet' or whatever wouldn't mean anything in terms of whether that oppurtunity came up... I wouldn't force the situation in any way.
However, I voted 'NO' because morally (and ideally) I am against it, and I would definately feel really bad about if afterwards. At the time people make mistakes though - especially if they are drunk... so I couldn't say I would be able to say 'no' if she really wanted it...