Many marriages in recent decades have ended because one partner or the other acknowledged that they were, by nature, gay or lesbian. Often part of the process that led to that acknowledgment was sexual activities that violated the marriage vows of sexual fidelity. As a general rule, many consider these vow breaking acts to be less of a betrayal than heterosexual adultery, because it is understood that the marriage vows were in some way less than fully binding because they contradicted one of the party's basic nature. We now presume that gay or lesbian sexuality is part of someone's nature, whatever that is. One can't be expected to keep a vow that contradicts one's basic nature.
Many of the arguments that the affirmation of multi-partnered relationships is warranted, necessary and socially beneficial include the notion that some people are "poly people" and that to expect a "poly person" to uphold monogamous marriage vows is like asking a gay person to uphold monogamy with an opposite sex partner; it can be done, but only at great cost and difficulty.
Floating the meme that some people are naturally "poly people" into the culture sets loose a ready-made excuse for all sorts of adultery. My mother asks "What if every person caught in adultery can point to the fact that they is coming to grips with his/her essential poly nature, and hence this affair, while regrettable, is as understandable as Jake and Ennis' fishing trips on Brokeback Mountain?"
It matters whether this "poly" sexuality is a real thing, just as it matters that our scientific understanding of gay and lesbian sexuality points to a real difference. Yet, is there any real scientific evidence that "poly" sexuality is real?
My suspicion is that human beings are neither monogamous or polygamous by nature, but are so, by culture. Certainly, by nature, humans beings are capable of, and desire, sexual relations with many different people. Cultural rules and standards say that some desires can be appropriately acted upon, and others cannot.
In the absence of any real evidence of natural difference, we can only conclude that people who are "poly" are proposing a different set of cultural rules that govern our sexual behavior. To make that argument that they are compelled to a different set of rules because they are, by some nature, different, is a deception, first of themselves and then of others. Cultural rules based on that kind of deception, and false understanding of human beings, will fail and cause more harm than any good they might do.
And the immediate harm will be setting up another reason why it is somehow acceptable to violate one's vows of fidelity in marriage. In a culture already awash in infidelity, suspicions, jealousy, controlling abuse, and the abandonment of single mothers and children by fathers, creating specious justifications for adultery is socially irresponsible.