See, to me this has nothing at all to do with Dean’s interest. This has to do, to me, with Dean seeing Sam as potentially interested in Brad Pitt and nothing more. To be fair, there is a decent amount of evidence that Sam’s sexuality is not straight.
I mean, the fact Brady and Sam had “a history” and Crowley called him Sam’s “demon lover” and the fact in the episode where Crowley was trying to protect Sam from seeing Brady, when he took out most of that demons’ nest and lied about working with Brady, he said Brady was his “lover in league against Satan”.
The fact this season Sam said he dated “someone bendy”, which is not gender specific. The fact his concern when hearing that Bobby made a deal with Crowley was to ask, “Did you kiss him?”
The fact he’s never, ever said he doesn’t like guys. Dean has said it at least 5 times, usually defensively to a potential threat, but that’s still a statement of sexuality Sam has never done. Also, Jensen and Kripke consider Dean straight. It’s honestly weird to me more people don’t ask about Sam’s sexuality.
I think this scene is actually just a little indication that Dean knows Sam is not completely uninterested in guys. I don’t think he’s just teasing him when he says it, I think he is actually asking too.
Sam has also summoned not one, but two crossroad demons— demons that are meant to take a form appealing to the summoner— in male vessels.