Feel sorry for the husband in this case. Cheating is bullshit. Everyone gets a happy ending, except for the guy who is providing for everyone else. What a garbage story.
I'd argue that he's the happiest of them all, since he doesn't have to suffer the crippling doubt and self hatred that plagues the two women.
Well, in the moment, this is true. And the next moment we consider is him discovering that the person he has entrusted his household and a considerable chunk of his life to has been lying in his face for a long, long time. No matter how you look at it, cheating hurts.
It's Sei who will probably lose. Ryouko chose the traditional married life, over an alternative life style, for a reason. So, if she is put on the spot, and forced to choose, who do you think will be kicked to the curb. Besides, poor Sei will probably be pushed aside, and forgotten, once Ryouko has children.