Let's say that his wife isn't better either. I get that she's a woman "marking her territory", but telling this sexual history with such details to a girl, that already told her that she isn't a slut, doesn't make her a best person either.Not bad.
But you'd think a 40 year old married man would know how to act like...a 40 yo married man. He's already a serial cheater which means wife at least suspects him. And that dude brings a young blonde to his wife's home, not telling her shit, then cleaning out her dead relatives stuff like it was garbage to accommodate her, right before he has her sleeping on his dick for his wife to see.. And after all that he is acting like it was a minor thing, like not cleaning the dishes or something. I will cook for her and everything will be ok. He is lucky he still has a couch to sleep on.
Protag is already flawed with all the sex addiction and serial cheating shit. No need to actually make him a drooling retard on top of all that.
Having said this, I agree with you that MC's personality doesn't relate to his age. He's very immature, but is way better that most of MC's around.
Until now, the younger one is the wiser. Let's hope that the most probable corruption path won't change that. Maybe as other pointed out she will be the glue to this couple and in the end it will be a triangle.