Sorry to argue here...But why do you keep arguing about the sex scene? The sex scene is not the point, the point is that Blair is totally insane for going back to Chuck after everything he did to her.
Just the last episode of last season, Blair chose Chuck and therefore prooved that what everything he did in the past is forgiven (and he did a lot!) Then he goes to slap her in the face (metaphorically, because we know Chuck is not violent, his hand is just attracked to glass and glass seems to be breaking when he hits it and the glass seems to want to fall on Blair's face) so he slaps her with insult such as : 'I don't want to be Mr. Blair Waldorf and he degrades her as per usual. The next thing you know, Blair is betting all the hard work money her mother made in her entire life on him in Paris... THAT IS INSANE!