Crowley watches Aziraphale's gesture with something akin to wonder. Wonder at being loved by Aziraphale----or perhaps being loved at all---so entirely, so sweetly and with no alternatives behind it. Aziraphale is so genuine, and has always done things because they are the right thing to do. He must believe loving Crowley is the right thing to do.

It's a rather impressive responsibility, having to prove you're worthy of such a pure love. But Crowley will. He's always tried to prove he's worthy of Aziraphale in the past, he can do it now, now that they're here.

He leans down to press a kiss to the angel's neck, then to his collarbone, with the same sort of softness that he gets from Aziraphale's lips. He wants to know him everywhere.