You'd be surprised. The fact is most men who settle down with a woman actually do want to settle down with *that* woman, they're bored of playing the field and just want a comfy life partner who they have sex with. The Lying Cheating Insatiable Husband trope is mostly a fiction, in reality it's women who become restless in stable monogamous relationships because their entire sexual mentality revolves around a constant nagging sensation that they *might* be able to do better.
For a man settling down is the desired end-state, a comfortable retirement from the chores and stresses of dating. For a woman settling down is merely settling, an acceptance that she's peaked and can't do any better, a thought so depressing to most of them that they'll fuck around at the drop of a hat. Every straight couple I've known in "open relationships", the guy has gone outside the marriage once or twice at most and doesn't particularly care for it, while the woman becomes a monstrous whore out shagging a different man every weekend with absolutely zero regard for the way she's clearly grinding her husband's self esteem into paste.
Never, ever allow yourself to be memed into allowing an open relationship, if your wife or GF proposes it either drop her on the fucking spot, or at the very least have the stones to refuse outright and keep a careful eye on her from then on.