This is the moment for Castle, where he realizes that Beckett is it for him, that he needs to break up with Gina because he knows their relationship will never compare even to small moments like this with Beckett. This is where the magic of their relationship starts. And Castle realizes that what he wants is right in front of him.
perhaps, I thought it occurred much earlier. in series 3 as opposed to series 1 and 2 there is no great confidence he wil go out with him.