Well, now they're here. Half-naked in a bedroom and no one in the universe to stop them. Crowley feels something shoot through him that he can only think must be like performance anxiety----after all, this is happening, and he's leading it, and he can't muck it up in any way, lest he totally disrupt what they've built so far, and....and...

Oh, what the hell is he being nervous for? This is Aziraphale, the person he's been beside for six thousand years. The person he, despite all hereditary and societal implications, loves completely. He can only utterly ruin the greatest relationship he's ever had in his whole life if he does something wrong, what is there to be nervous about?

He turns and leans in to kiss Aziraphale. Kissing is good. He can do the kissing, he's gotten less nervous with kissing Aziraphale.