He really hasn't thought this through, and even though it's tempting, he resists and simply goes with it. If he thinks too much, he doesn't know what he might do, and he doesn't really want to second guess whether or not they ought to be having sex.

His angle makes things a bit difficult, but he takes advantage of what he can, mapping the side of Watson's neck, biting gently at his jaw, tugging his earlobe between his teeth and sucking lightly at it. He gives a small puff of frustration at not being able to attack Watson as he'd like, and vents this by reaching for Watson's hair, so he can pull him into a deep, hungry kiss.

There really is little he can do about his growing erection, but he's not embarrassed by it; he only hopes that Watson won't decide that they're not ready for this yet.