It's a good thing Jean wasn't drinking the champagne, otherwise it would have been wasted at that moment. Instead his face is both shocked and very red.

"LIKE HELL I'D DATE EREN!" That's the main issue with the suggestion, clearly. "There's nothing that come between Mikasa and him so 'adding a third' is out of the question and like hell I'd 'share' her with Eren. Mikasa's not something you can just go around sharing anyway."