I dunno why, but I was just thinking about all of this again (since this series is so silly and makes no sense).
What if she really isn't doing a guy and there is another girl in the room with her at night. Sure, he hasn't seen anyone come in or out, but a huge to do is made about hearing how he hasn't heard a male voice, just a female voice. What if, the other girl is the neighbor and is one of their old friends from the little town they all come from and this individual is helping the FMC with whatever it is she is doing (livestreaming, VA, ASMR, or straight up just sex), but no male is involved at all?
We saw a picture at one point that had 4 people: MC, FMC, and two other girls. We have met one of the other girls which is the glasses girl who wants to bite him at the end of this chapter. What if the other girl in the picture is the person who is next door and our FMC doesn't want him to know just yet because then they would need to explain why she is there and because she doesn't want to lie to MC she can't do it yet.
And what if the kiss from when they were younger was just her wanting to try it and see if she had feelings for our MC or not? And she realized right after the kiss and him running away she had feelings for him and nothing ever came of that again (this one seems way too stupid in my opinion).
We know FMC is into MC based on the way things have gone so far: she went into his room and slept there overnight, she was touching herself while smelling his scent, she kept all of the things he ever gave her, so on.