Crowley is an admittedly sexual creature. He's not so lead by his dick that he makes all decisions by it, but he does find that there's a sexual edge to much of his thoughts and he's rarely one to pass up an opportunity for debauchery.

His mind so easily fills in the blanks that Blake is feeding him and it's no small coincidence that Blake fits into these sort of blanks more easily than most.

The rise and fall of his chest is more pronounced as his body reacts to Blake, blood slowly pooling downwards.