The "open marriage" concept wasn't a particularly good idea in 1972. It's even a worse idea now. So why dust off this discarded content? Ultimately, calling for "open" relationships tends to be about having your cake and eating it too: wanting to have all the stability and emotional support of a trusting relationship without any of the commitment.
But unfortunately, reality doesn't work that way. Commitment is necessary to have the level of trust needed for a healthy relationship. Infidelity damages that commitment, perhaps not visibly at first, but inevitably the damage is done. That's why "open" marriages are, despite some high-profile examples, not that common. We're wired for monogamy, and that's why we're happiest in monogamous relationships. The "open" marriage is about as durable and dependable as an AMC Gremlin, and both belong in the junkyard.