Well said. Though of course another aspect of those social institutions was hierarchical stability: if a peasant married a nobility, had a child, and then gained claim to this family's assets then all would go to pots soon. On the contrary a marriage between two noble families would create political bonds to benefit them both. In fact the notion of "romantic love" didn't really exist until the renaissance (though unfortunately now we know pretty well this notion is also quite BS and against human nature anyways). Now with productivity largely increased and ordinary people having much more economic power such an institution has no reason to stand. It's just that social changes mostly occur slower than technical/economic basis for them.

And indeed, a relationship has to be based on mutual agreement and trust. If both sides agree this relationship to be open, then there's no problem at all. It's only problematic if they reached an agreement when starting the relationship, but later couldn't adhere to it. If no deal, then no relationship. If there's a deal, then you'll have to follow through.