What would u do

I’ve been dating someone for 6 months. We have not made things official but I would always ask if he was still dating others and he would assure me he was not. Yesterday someone messaged me on social media telling me we’re talking to the same guy. They had sex. I called him and he confessed saying it was something casual and it just somehow happened. He wasn’t taking her seriously and the girl was making it out to be bigger than it was. He says it’s not cheating since we aren’t bf and gf. He said he never took her out like he does me or invested his time on her like he does me. We’re together every weekend. He then said he likes me and will make things exclusive with me now. What would you do? Because i do really like him I’m thinking no contact and see if he pursues? Am I incredibly dumb?