I totally understand your points, and they are there, also there is if you want your son to get along with your "partner" is encouragig him to flirt with her the right aproach ?
Hmm, probably not. But Charles seems to believe that the best way to get on a woman on your side is to compliment and flirt with her. He wasn't angry when she complained to him about the MC spying on her, so he definitely gives the MC more leeway than most other men would around their wife/girlfriend.
and due to all the side effects of his meds, he feels guilty and to some point he (Charles) feels more a Daddy daughter love than a love to a woman, also he expresses that he want to give Beth the satisfaction to her urges and dreams, about children he won't risk having one with her because he feels that he oves his son.
He did mention something about their relationship being more "friendly" than romantic at this point and that he isn't sure how much longer he can hold onto her but she
is still his girlfriend at this point and he hasn't given his explicit permission for the MC and Beth to start a sexual relationship, they did that behind his back. The MC is probably a bigger offender than Beth as he's always the one that initiates things but she doesn't exactly put up a lot of resistance when he comes on to her.
Honestly, considering how the relationship with Beth has played out so far, i would be a bit disappointed if Charles found out about what they've been doing and was immediately OK with it. At the very least, he should be angry over the betrayal of trust. No matter how you look at it, Beth is still cheating on him with his "son".
Also, i'm pretty sure his lack of children with Beth has nothing to do with the MC. He wanted to have children with her, but he simply wasn't able to get her pregnant. He's probably sterile or something.