Well at this point it really doesn't matter if Sangmi or her Mom will have sex with Douche or any other guy. Even if so, it is neither cheating nor netorare.
MC already has had sex with several women including Sangmi and will continue to have meaningless sex with several women as he wasn't able to confess to Sangmi when she asked him straightforwardly what he feels about her.
I'll keep reading but am a bit disappointed. Could have been a light romance story but instead it feels more like one of those thousands japanese harem stories.
Well at least it didn't turn into cheating or netorare.