I guess she wants mc to be a sex friend since the cheating is normal in japanese culture i mean u can have mistress but yeah you must not have romantic feelings towards that person.. i know its a messed up culture.. but still its their culturestupid sensei , seriously she cant just do that, why you're not confess your feeling in the back then.. I mean like dude c'mon the MC is a virgin and she just gonna take his virginess like that? That's suck af