Obviously, I think Castiel would agree to have sex with Dean regardless of his own desire as well. Mostly it's because I don't believe Castiel understands love beyond agape...yet. This (in addition to the ridiculous codependence Sam and Dean have going) is the reason I believe Sam to be an integral part in any relationship Dean has - Castiel especially. Sam is one of the only people Dean will pretend to listen to, and Sam is also the one person that knows Dean well enough to translate his relationship-related ridiculous adequately. I also think he would have the patience to help Castiel figure out the emotions as the blossom. That's a whole other discussion that I need to have with my meta brain later.

Sweeter stories (for me anyway) have to be perfectly balanced and tonally correct. I have a hard time finding something that really resonates as "My Dean" and "My Castiel" because they aren't. There are subtle shades of difference that are sometimes enough to pull me out of a story altogether. This is something I can TOTALLY see happening though, messed up as it is. And, oh yeah, deliciously achy and fucked up.