This actually isn't necessarily about being unconventional. Women get bored with monogamy long before men do, even if they really love the guy, and like the coziness of monogamy. This is undoubtedly why most marriages that open up do so at the behest of the woman.

The Bored Sex

Maybe Her Low Libido is a Sign of Boredom

As you noted, opening up can actually benefit a marriage and strengthen the relationship, if it's undertaken well and with good communication. Couples can address boredom in other ways, but it is becoming more commonplace to see ENM as a way to do that.