I don’t care who “buys it” including you. I have a very happy relationship with a husband, another male life partner, a girlfriend and occasional other play-partners. Polyamory has enriched our own personal growth as well as our relationship with each other immensely. I don’t need you to understand that in order for it to be true.
Monogamy is not a natural human state and is a relatively recent arrangement.
“Given the fact that 80% of early human societies were polygamous, why did later populations become largely monogamous? Science has no answer to that, apparently, although there are theories, as you might expect. One of them has to do with the “two parent” advantage to monogamy in caring for the young.”
“The Na of China, farmers in the Himalayan region, live without the institution of marriage. Na brothers and sisters live together their entire lives, sharing household responsibilities and raising the women’s children. Because the Na, like all cultures, prohibit incest, they practice a system of sometimes furtive, sometimes conspicuous nighttime encounters at the woman’s home. The woman’s partners―she frequently has more than one―bear no economic responsibility for her or her children, and “fathers,” unless they resemble their children, remain unidentifiable. This lucid ethnographic study shows how a society can function without husbands or fathers. It sheds light on marriage and kinship, as well as on the position of women, the necessary conditions for the acquisition of identity, and the impact of a communist state on a society that it considers backward.”
https://www.amazon.com/Society-without-Fathers-Husbands-China/dp/1890951137
