The familiar statistics are that fully half of all marriages end in divorce, and closer to 60% in cities, (though it may have stabilized or be in decline with the economic slump—divorce is expensive), and there's no telling how many of that remaining half remain in unhappy marriages for their children or religion or fear or whatever other reasons people are averse to messy legal battles that still carry harsh social stigmas in many areas. And the statistics for cheating are pretty high, too, though certainly under-reported. The outlook for happily ever afters is pretty grim, but there is an alternative to perpetual singledom and almost guaranteed mutual annihilation.
Ethical non-monogamy means stating and honoring relationship agreements about actions and feelings permitted with others outside the relationship. (And being honest with those others about being already in a relationship and being open about those agreements and how they will affect the other relationship.) In short: be open, be honest, and secure the consent of everyone involved.
It may seem an alien concept, but consider the continuum in monogamous and monamorous (married and dating) relationships that permit or prohibit each partner to have close friendships or even casual friendships with people of the opposite sex or with exes; to casually or aggressively flirt; or to kiss or to dance with others. Some have agreements that permit hugging or hand-holding with others. Some have occasional threesomes; they may call themselves monogamous, but I like Dan Savage's word monogamish.
There is no one way to do monogamy or monamory, and the same is true for non-monogamy and polyamory. We each have different relationship wants and needs. Our relationship styles may be said to fall along a continuum rather than being polar opposites.
Neither monogamy nor non-monogamy is THE ONE BEST WAY™ for everyone. THE ONE BEST WAY™ is for everyone in any relationship to think about and discuss their wants, needs, and expectations. Cheating for one person may mean a partner having an intimate email exchange with another or it may be PiV intercourse; it may be flirting or having an affectionate friendship with another. Stating and honoring relationship agreements is necessary for ANY relationship to thrive. Polyamory for some is a no-holds-barred free-for-all, but others may only permit casual secondary relationships or for the couple to date other partners together.
It doesn't come naturally. Communication takes practice. And introspection can be scary. But valuing and practicing the two will make you a better person, and you will have better relationships. I promise.