I'm a full subscriber to monogamy, and I don't even think that is true. I think the vast majority of us aren't in a mindset where we could imagine that being our existence, but I do think even in a monogamous construct we find amicable ways of sharing and exploring with others that, some would say ideally, should be sought out in a partner. Sure we're talking about sex here, but how many degrees of jealous/insecure partners could make pretty valid points about things they wouldn't like their significant others doing with others, including but not limited to sharing humor, platonic affection,coworking, dancing, etc.,?
I also think to suggest the "goodness" of a relationship, or by proxy, the value or worth of a partner is determined by a lack of desire to share or explore sexually with someone else, is well, childish, or at the very least, a sanitized and fairy tale-esque way to look at relationships. Monogamy is cheapened by likening it to a pleasant fog that blinds you to all but your partner. I've always thought the true value of monogamy was a consistent and conscious choice to honor a decision/commitment even if a "better deal" comes along later. If those in open relationships have found a way to compartmentalize their sexual activities away from the other socioeconomic and cultural aspects that define a committed relationship, then more power to them