I believe the concern here is that if Gary is half as accomplished as we're hearing, he should be used to nude model drawing by now. I mean it's possible he just never did much of it but by the time you've done it even a little of it, your mind just kind of goes into a different mental state.
Of course I think it's a bit excusable considering the context is a bit unusual. Dude's been lusting after her since day one after all.