Desire smiles into the kiss; Aziraphale is pinned, between the comparatively slight pressure of their form against his and the immense force of water outside the glass. The angel isn't going anywhere.

They mouth at him, exploratory; the adopted form does the job, gets them an in, and keeps Aziraphale from resisting as much as he might. They rub against him, angle in and drag their teeth over his neck to get a better feel for him.

And oh, there's fire underneath the angel's bumbling exterior and all that fear of his own wants, and now that Desire's got a taste of that heat they want to reach in, warm their hands on it.