A high number of previous sexual partners is a proxy for 'highly attractive' - people who are hot get partners more easily. It's likely that someone who is hot is going to be more willing to leave a relationship that they're unhappy in because they'll be confident they can find a new partner. Someone who is less attractive (or considers themselves less attractive) will stay in a bad relationship so long as it's better than being on their own.

I would argue that means the person who has had more sexual partners makes a better spouse. No one wants to be stuck in a relationship with someone who's unhappy but unwilling to let go. That's toxic af.