Exactly this. It's much different finding out something like that because your partner was upfront about it, especially when they're still good friends. It's entirely different when you find out through the grapevine that your partner slept with their best friend after said partner lied about it and denied it.

Yeah, I know I would still be insecure, especially if they wanted to hang out one on one, but it would be a whole lot easier to come to terms with if he was upfront about it right away, instead of hiding it. Hiding it would make it seem like he does have feelings, and he didn't want me to find out so he could spend more time with her, hoping she'd come around.