A social structure that evolves with the people in it. The main problem with marriage is that it’s built on very rigid, very old ideas and people tend to lose their heads when it comes to working on theirs.
Whatever the social structure is, we’ll probably call it the same thing we always have, family. The vast bulk of families will be heteronormative simply because that’s the way genetics is. Roles will be more fluid, but they’ll more or less look the same. The stereotypical Christian household is a very appealing ideal.
I don’t see family planning as the big issue in the replacement of patriarchy but rather in the practice of divorce. As long as we’ve this social norm of putting children up on a moral pedestal and forcing parents to make grave sacrifices for them against any sort of logic or reason, the institution of family won’t be able to move on past roles.
I’m hoping the culture of fear that has developed around parenting will lift soon.