That one I get even less than “Cas is innocent”. I mean, being a virgin doesn’t make you ace; it just means you haven’t had sex yet.
We don’t know much about the sex lives of angels (and those are words I never thought I’d ever type), but Cas is a soldier of the Lord. He’s been on active military duty pretty much his entire existence. As he said in “Free to Be You And Me”, that doesn’t leave a lot of opportunity for “cloud seeding”, especially if you don’t want to shtup your fellow grunts.
But that doesn’t mean he’s never wanted to. If nothing else, what about 2014!Cas’s orgies? Seems like pretty convincing evidence to me that he’s got a sex drive, and a rather hearty one at that.