One time was enough for her to give up on her BF. I think the author wanted to go for more pages but rushed the ending, title says M Girl but she was quite assertive at the end pushing the other guy down almost going yandere the cover with the collar was a lie or just the idea he had in mind?
Boyfriend kind of sucks in this, so the NTR is weak. Also, she's slept with the senpai a few times before giving up on the boyfriend. In her words, she continues with the sempai since she's able to be herself with him rather than trying to appeal to her boyfriend's tastes.