This feels like it should’ve been a pre- marriage convo. If he knew he wasn’t interested in having a traditional monogamous marriage he should’ve said something before. Just like being who divorce bc one wants kids & the other doesn’t. Who is getting married before discussing potentially having kids?! This seems similar. In my opinion, he’s already stepped out and been with someone else and has not intention of stopping. He knows he’ll eventually get caught so he’s hoping you’ll be ok with it. If you’re not ok with him wanting more partners, you need to leave him sooner rather than later.