I think that time Nagatoro grabbed senpai's dick is when she became so overprotective of him. She realized if the other girls found out about his massive dong that he would become so popular that he would be unbullyable.
That's the most textbook pissed off wife face I've ever seen.
And something tells me senpai is going to be seeing that face all throughout his life, the lucky bastard.
Leave it to the slutty girl to know what's really going on, or does she ? She's that "does that face means she knows or is it just her derpy nature?" type of derpy looking character.