Why would you want to waste time being a placeholder for someone who will jump at the chance to hook up with another young coworker that shows interest in him. How could you trust that he won’t fuck his female friends when he hangs out with them? He did it while you were out of town. Glad he told you because you would’ve never known. But he still sees and interacts with the girl regardless. All you would be doing is accepting him cheating on you and waiting for the next time that he does it. You could do something else with that time.