I don't think it makes sense to feel betrayed, personally. This is a mistake he made in his past, and it sounds like it was a much bigger one than the "physical affairs" that he told you about already. It seems like he had to build up a certain level of trust with you before he could bring himself to tell you. I think it's pretty normal to only share harder truths about ourselves over time. If it's something that directly affects you, and he didn't tell you at the outset, that's a big problem. But this doesn't directly affect you at all. So, why feel like he should have told you everything about his past all at once?
On the other hand, while I don't think it makes sense to feel betrayed, I DO think it makes sense to be upset by this news. It sounds like he's a big old hypocrite and a liar -- he's kept this huge thing from his wife (if he won't tell you whether or not she knows, it means she doesn't know), while giving her hell (I imagine) for doing the exact same thing. Of course it doesn't feel good to find out that someone you love hasn't been a particularly good person.
The real question is, does he understand why his actions were wrong? Has he learned, grown, and changed... or is he still the exact same guy, still completely capable of lying to you on a huge scale at any time, and he just doesn't happen to by lying to you at this particular moment (that we know of)?
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.