I don't like it either, poor guy has to witness other characters getting it on while he is left in the dust with a crazy bitch, I assume that the author thinks that this is the most interesting pairing, Milk Ninja would be too "easy", she most likely would be down for anything and would make an ideal partner given how much they share and can reliably work together, but man I hope she actually changes within the next 10 chapters, otherwise is just a neverending loop.
I get the author seems bent on keeping a tense atmosphere in this Manga and I commend his ability to give the MC overpowered abilities while also making him look very fragile, but right now it doesn't feel like much progress has been achieved, he needs to leave to this world after all.