My husband and I have been together for nearly as long as Nick and Kayla. We've been best friends since basically the second we met. At this point I can't imagine my life without having him there as my partner, companion, and confidant. Our marriage is a safe haven where we can be open and honest and vulnerable and share things that we can't share with the rest of the world.
We've actually been discussing this very issue while following the Balldo spiral (Mr. Sniper is also a former viewer), and violating that safe haven is completely unthinkable to both of us. Like we can't even fathom having such a breakdown in communication that we'd consider opening our marriage or swinging or any of the other degenerate shit Nick and Kayla got into to "fix things". If my husband was ever unfaithful, it would be such a violation of trust that I wouldn't be able to come back to the relationship at all, let alone settle for being the one he decided to keep.
I go back and forth on how sympathetic I am toward Kayla. I don't think she's very intelligent or strong willed, and she was very likely an easy mark for Nick to manipulate. But at the same time, she let this happen to her kids, and the fact that she's still there after all of this makes me believe that the swinging wasn't entirely Nick's idea.