Sam turns his gaze on Zachariah, very obviously uncowed. It would take a better Beta (read: Dean) than him to make Sam stand down.

Then he smiled, still not having taken his hands away from Castiel. "You misunderstand," he said after a moment. "After all, when it comes to these sorts of things, what someone 'wants' is very different from what happens, is it not? While I may not, currently, want to marry Castiel, I'm not ruling out the possibility that that will change later on. So I think I would rather stay."

He paused, then turned back to Castiel. "As long as you're happy with that, of course."