Non-monogamy makes sense for many people, IMO. Something like 30-60% of married people commit infidelities, and something like 50% of marriages fail. They and their relationship would be better served by opening things up and talking honestly about their desires, sexual and emotional.
That said, monogamy is also a valuable experience, one that, in the right context and between the right people, enables an intense emotional and sexual bond.
At the end of the day, there is no right answer. People need to work on building open communication and better relationships first and foremost. Without a solid base, non-monogamy can easily result in disaster, just as much as monogamy.