I meant Masahiro for the "ojisan's" name.
I didnt quite got that part either because to me it would only be natural for espouses to grow apart after an infidelity.
Exactly! It's hard for a couple (that promised each other monogamy) to get along as before after one partner's trust has been broken so horribly by the other. Even after loads of marital counseling, sometimes couples are not able to maintain the same momentum and often divorce. And if he was referring to Ikuomi's behavior prior to the whole infidelity debacle, cheating on him is not the correct response if she felt neglected or unhappy!
I didn't get the whole, you hurt your ex-wife by trying to protect her or something that the Ojisan (Masamune?) kept going on about. Is it explained anywhere in the book, since, from my end, it seems the other way around. Ikuomi's ex-wife cheated on him with a guy who was like his brother and gave birth to that guy's kid and left her husband to take care of it. While I do not approve of the way Shougo was treated, I fail to understand the whole Masamune is angry at Ikuomi for hurting(?) his ex-wife.
Maybe I just glossed over the explanation because I was, for once, very happy that Kazumi (seme) was willing to step up and take care of Shougo, regardless of the feud between the fathers, and give Shougo self-confidence and an outlet besides Kabuki that would help Shougo to define himself.