A couple is supposed to "belong" to each other.
For example how would you feel if you knew that he was buying expensive clothes or jewelry or paying bills for some girl that you didn't know that wasn't a friend or anything just some random girl. Not knowing whether or not he's having sex with her just that he's paying her bills and buying her gifts? Especially if he was coming up short buying you nice things taking you out or paying the bills on time? I'm pretty sure you would feel some level of betrayal for that.
From what I gather you might be from England or Australia the only thing I know about those two countries is that they are very gynocentric. (aka "woke") Not a great place for a man to live in the current state of things, that said it's not too much better here but it's not quite that bad yet.