What would you do?

So I’ve been seeing this guy for almost 3 months now and he’s a real gentleman, and we’ve been on numerous dates. He’s taken me to look out points, restaurants, walks in the park, and sometimes we just chill at either of our places. We’ve fucked a couple of times and he keeps wanted to do it more (which is totally fine with me). But then the other day he accidentally sent me the wrong message over snap. And it turns out that he has a girlfriend and they’ve been dating for a couple years. And he tells me that it’s an open relationship, and that he just ‘forgot’ to tell me about it. I’m not sure what to think or what to do.