It seems like you should get your sequencing together. If you are staying for the kids, you want to do good modeling for the kids. I admire this guy for not pursuing you. I think he might value the institution of marriage more than you do. The question of whether humans are monogamous has been battered around for centuries.
People vary in their interests.
Usually after a few years of experimentation, people settle down and are comfortable with one person. There are many different dimensions of the question you asked, but a person can find someone else who won't cheat on them. A husband hanging out in gay chat rooms is looking for something. I would have a discussion about this because it can spin out of control. It might be playful experimentation or he might be bisexual.