I'm having real trouble thinking through this properly

A few months before my bf even met me he got drunk and slept with a flatmate. Nothing else happened following that, but they're still good friends (all their flat still is). I feel really jealous, but am I right in thinking if anything would have happened/was going to happen, it would've happened before we started going out/ we never would've gotten together if he was preoccupied with someone else? (He went to her birthday - all of his old housemates were also meant to be there- straight after staying at mine for my birthday - bought her a present and left mine a few hours early - plus he decided he'd rather go out with a group of old friends including her than go out with me a few weeks later). We're no longer together and I'm sure he'd never cheat, but I can't help but feel upset about this

I think that since youre not together anymore that you should not think about it. It seems like this guy was kinda of a jerk. Really just let it stay in the past. If was such a stupid person that he couldnt try to make you happy then he doesnt deserve you