First of all, this is another secret she’s keeping from her partner. Second, it’s obvious she only has eyes for her bf but the senpai being very close to the bf is like an excuse for her to keep these soft-cheating secrets. If this was a josei, her walls would crumble allowing the senpai to cross the line. And since she’s socially inept, the senpai would somehow convince her it’s for the sake of loving her bf. If she doesn’t fess up to what happened before the story ends I’m gona be upset.