I'm not mad, that he stayed faithful to his wife. It's the honorable thing to do. But I will get mad, if he doesn't take that as a wake up call. I mean there is a good thing going on between him and the lady lord. He should confront Ami. And with confront, I do not mean asking her, if she's cheating on him. She'll just stiop for some time. Life time he loses. He can use his superpowers (climbing walls and stuff) to tail her or better hire a P.I. Get some pictures, to actually give himself a "I can fuck aroung as much as I like"-card and get a divorce while he does so.
But I do believe she actually doesn't cheat on him. In regards to the "she is also a summoned beast" theory: Maybe she is the Phoenix and wants to test him.