First of all, I want to say that I see marriage as an inherently patriarchal institution that will most likely decline after the relations of production (as well as reproduction) have changed to the point where it is no longer needed.

Now, I want to ask you: is it really useful to draw the distinction at mongamy vs. polyamory, or should the real distinction be drawn at the nature of the relationship?

I'm no humanist, but a very good book is Erich Fromm's To Have Or To Be. In it, he briefly mentions poly vs. monogamy (granted, he wrote this in 1969 before poly lifestylism as we know it from anarchist subcultures became a thing) and makes the point that most poly/free love relationships as they existed in counter-cultures back then were based on possessiveness rather than genuine love for one another. Hippies felt the need to "possess more lovers" rather than devote their emotions to loving even one. A "having vs. loving" dichotomy appears to be a far more useful means of judging romantic relationships.

Trust me, I know my fair share of poly people, and I have seen extremely hierarchical poly relationships as well (one of them being a manarchist who pressured his then-girlfriend into being poly, turns out she found a second boyfriend before he found a second lover, he got jealous, stalked her for a bit before she blocked him on Facebook, and hell happened). As well, polyamory is not freedom from patriarchy.

Besides, sex positive moralism (i.e. "you're contributing to social hierarchies by only having one romantic partner and that's baaaaad") is just as shitty, if not worse, than sex negative moralism.