It's one of those things where-- cruel as it may be to admit it, Benedict really didn't think Colin had it in him. But he clearly does, to a near alarming point, and Bene makes a little sound of surprise at the feeling of Colin's hand down his pants.
Not that he's protesting, he just hadn't expected anything so forthright.

The sound changes to something like a sigh, his hips tensing and drawing nearer to the man above him.