Dean places so much importance on sex, or he has, and it seems to be a fundamental aspect of humanity and, by extension, free will. Castiel's looking to flesh out his free will, to take what angels have so often refused to take or simply been uninterested in. He's seen the extent to which angels can resist the idea of being their own... their own being, and it seems to him that sex, at its core, celebrates an individual's right to pleasure for the sake of it. So yes, he has thought about it, in an effort to drum up his own interest. He even kissed Meg, and that had been enjoyable, and then he kissed Crowley, and... kept kissing him. He understands the appeal now.

He thinks he's still doing a good job, judging by Crowley's moan. He only has one real idea of what Crowley might like -- pain -- and so he finds his neck again and bites, harder this time, moderately more sure of himself than he was before. He quickens the pace on Crowley's cock, and he grips his hip too, fingers pressing into him.