It's how many humans naturally behave.
Many humans behave in a serial monogamy manner - which is not poly. Many humans behave in a covert polygamy manner - which is not poly. Whether there is very much left after that which matches US polyamory, I wouldn't know...
Recently, LWers Will Ryan and Divia Melwani (now Will and Divia Eden) were married, with Eliezer Yudkowsky officiating.
I've been to 40+ weddings in my lifetime, and this was my favorite ceremony yet. Here is the video, and below is the transcript of Eliezer's... what's it called? "Blessing"?