O, I'm just ruminating about my own situation.
I'm in a long-term marriage with a kid. My husband's girlfriend identifies as poly, but has never been married or in a long-term relationship. She definitely wants a primary (which puts a lot of stress on my husband). It seems that she would be able to get her needs met better if she just approached the situation differently, but who knows!
I do thing being single is inherently poly, because most people date lots of people until they "settle down." But I, personally, would;t date a married poly guy if I wanted a primary partner, it seems too painful.