If you watched the season 5 finale (no one would blame you if you didn't. I sure as hell didn't) then you know that Serena recorded her and Dan having sex. And it good teen drama fashion, it wasn't revealed privately but put on display during an event/party for all to see. Thus, triggering Blair to slap Dan and say that he acts like he's so much better than Chuck but he isn't. Later, Chuck says that he always thought Dan was the better man and Blair says, "no one is better for me than you."
Um, what you're talking about was speculation before the episode ever aired because everyone found out Gabe Saporta was going to be on the show.. But in reality, after Dan smashed Serena, he found out that she lied about the Chuck thing (except, it wasn't even really a lie). They had sex.
I literally gagged.
such a mess.