I feel like the husband just had a lot of insecurities and the affair was just a consequence of this. Like, he liked the other woman, because she seemed helpless, so he could feel like the protector in the relationship. And since the MC is more independent and confident, he didn't feel like he was needed, or something like that.
╮( ̄▽ ̄)╭
Men that feel insecure with self-assured women are going to be trouble sooner or later, that is, if they don't learn how to deal with their baggage.
As someone from Brazil, the idea of debating if flip flops are nice to wear or not is so weird, lol. If I could I would just use flip flops in almost every occasion, aside from heavy rain or really cold weather.