Why isn't he using his trickster magic? Because unwrapping presents is the best part, duh, or maybe also he hasn't internalized his powers enough that he skips things like taking clothes off the manual way. It would've been nice to think of that before, but he didn't, and anyway, now he's thinking about using his powers in a very different way. Cas wanted to get with both of them, they both wanted to get with him, but Cas kicked them both out. How would he feel to know what they're up to? To know that this time he isn't invited?

He won't make Cas into a fly on the wall, but he can make it so Cas can hear every little thing they do.

but now he's not thinking of much except that's a whole lot of skin he gets to look at now, and he runs his hands over her thighs before he gets on with the show. He maneuvers out of his pants and resettles himself between her legs, letting his eyes rake over her again before his hand follows, cupping one of her breasts.

"You sure you're ready for me?" he asks, his grin lopsided but the dark intent in his eyes doesn't let it stay playful for long.