Not a dry hump, but definitely not a good move by the girlfriend. You can even tell he doesn't like it by the way he throws her to the side. She shouldn't have done that so soon. Give him 30 seconds to revel in the victory. It is about him; he is at an age where it is okay for it to be about him. Soon enough, in ten years or so, it'll be about "them" and possibly their kids. Actually, most likely it'll be about him and some other girl (or maybe a guy or a trans if he goes that direction).
And whoever mentioned the thing about spoiling his parents' memory of the event... totally on point. Even though it is silly, it is 100% true.