1. Liz and Seb have had sex, and they want everyone to know about it.
Far from just telling the cameras at the start of the episode that they've had sex, Liz and Seb joyfully (see: "politely and respectfully") tell just about anyone that'll listen at the dinner party about their experience. It's written all over their faces, so Stacey and Michael clock it instantly — at least after they recover from the surprise of Liz and Seb jumping out from their hiding places and Seb as the "tickle monster". Seb admits to Liz that he's told anyone he's had a sitdown conversation with the news. And they're sexually compatible alright.