I agree that it isn’t necessarily patriarchal now (and by that word, I mean a part of a social system that is based in dominance hierarchy, where men have some power and primacy over women). But, we live in a country where marital rape wasn’t illegal in all 50 states until 1993. Before then, it was a considered a man’s right to have sexual access to his wife, whether she wanted that or not. Until well into the 1970s women couldn’t get credit cards or home/business loans in their own name. There are still golf clubs in the US where a woman can’t be a member unless she is a joint member with a father or husband. All of this informs how both men and women expect marriage to be, although a lot of that is subconscious — even if they intend to be equal partners.
I was happily married in a pretty egalitarian relationship for 20+ years and then we opened up our relationship. There’s been a huge and dramatic difference in the way that we relate to each other now that we are partners and not “husband and wife.” Since the roles are dismantled, so are our expectations that go along with them, and we have a much more cooperative relationship now. It wasn’t overtly hierarchical before, but it was in subtle ways, because that’s the cultural narrative of what marriage is. We didn’t realize how much we’d bought into that until we consciously chose to buy out of it. I’ve heard other poly people say the same thing.
And, I don’t doubt that there are monogamous married couples who have very equal and partnership oriented relationships, but I don’t think it’s the default. Women initiate 70% of all divorces and in more than one study have cited feeling controlled by their husbands as one of the primary reasons. Marriage has certainly evolved over time, but 50 years isn’t that long — really only a couple of generations, and 50 years ago nearly all marriages were highly patriarchal. I think most of them still are.