@LoudAndSqueaky From my experience as a guy, it's the way we are raised and taught how to be in order to be a man. If you think all our role models are generally like this, especially older generations and then socially from a child you're all following these kind of examples set from an early age it gets amplified and history repeats.
I was slightly different because I was mostly raised by women and I've never seen things the same, I didn't and still don't really connect with men due to this. I've come across, witnessed and heard some awful things that's normalized and encouraged because that's the way of the world, how men are and should be. Also, a consistent case of when the men are alone a different version appears.
Again, this is kind of scenario is more than jealousy, it's almost like a claim. Forgiveness if they do it because that's what we do, you know, it's normal, natural but doesn't work the other way around. It's shameful, he feels humiliated, betrayed, you were supposed to stay loyal.
@SuddenlyISee Glad you are seeing things clearly and making progress. If he does do something stupid like that call the police. Likewise, should he ramp up and you feel threatened or in trouble, call the police and you can keep him away.