Kinda mixed feeling about Riri's dad. He cheated, yes, deplorable, but he was the one to bring that up (at least that's how i recall it?), and who's to say that staying with unloved wife would've been a better choice (for all three of them). But his reunion with Riri felt really awkward and void of any closeness they were shown to have in flashbacks, and that tidbit about him stopping smoking made it look like he didn't care for Riri as much as he did for his new family. And yet he's still shown to have a spot for Riri in his heart.
From a religious perspective, he owes devotion to his first wife because he made a promise to do so.
From a secular perspective the marriage is for people in love and he shouldn't have to suffer with a woman who he thinks doesn't offer what he signed up for.
Admittedly some religions or religious people even among Christians would excuse his decision to leave his first wife. And many secular people reading this will be angry at him for abandoning our precious Ri-chan.
But i hope we can agree that made part of the problem is that it is a crime to register two marriages, even if both wives agree. Such laws leave him with only 3 options:
1- stay in a loveless marriage and probably suffer
2- treat his preferred love as a mistress with secret or an open secret.
3- divorce his wife and leave his first family for a new one with his second love
There is a fourth hidden option of trying to live together, but that would just be a special flavor of option 2.
Just thought I'd say that. I do not approve of his choice, but i do not think him unforgivable scum.