Castiel is being ruined in several ways here, most likely, but he isn't as aware of that now as he should be, or would've been before. He thinks of that as a good thing, that he's growing and letting go of the subservient mindset that prevented him from wanting more for himself, for wanting something different, and prevented him from seeing this as an acceptable avenue. What is there to be afraid of? There's a lot, actually, but he doesn't see any of it now, and probably can't; he's feeling too much physically and can't find room to be afraid.

The scratch of Crowley's nails cuts into the pleasure in such a way that the pleasure is easier to take, and he's definitely losing himself here. As Crowley continues, Castiel's hips lift slightly off the bed, chasing the contact of his mouth. He wants to do more than simply lie here and take what Crowley gives him, though, and so he places his hand on the back of Crowley's head (he's seen that done before). Following a precedent, both visual and now recently experiential, he tugs on Crowley's hair.