He’s well aware of my issues and said that although he would prefer if I didn’t hook up with other people, he’s okay with things remaining the way they are.

There’s a lot in your post. But the main thing is you are not and never will be in a serious relationship (imo) if you are seeing other people. It’s oxymoronic. Not sure if that was the intent to not when you felt trapped (prison) or what.

You absolutely do not have to be married to have a longterm healthy and serious relationship. It always has been and will be a state of mind and of actions.

Anything regarding marriage is just legal crap. The paper isn’t what creates the union, again imo.