Well, actually, one "sticks" to one partner in a so-called open marriage...it's about sex with others which doesn't make the others a "partner".
Still, I find this term a kind of excuse for libertinage.
RTH correctly wrote, that marriages have multiple reasons and - eventually - are contracts whose entering into can have various reasons - from economical and political ones to love and procreation ones.
Actually, I would plead to focus only on the own marriage and what one is willing to give to the partner (and oneself) as freedom outside the common life (and not bother about other people's married lives - except if they happen to the close family).
Edit: I just have started to watch a US webseries with Jane Fonda, Martin Sheen, Lily Tomlin and Sam Waterston..."Grace and Frankie" ... oh, I like it! (what an interesting plot)
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