I'd expect a lot more arranged marriages, especially since marrying a line disrupts the whole concept of romantic love. New potential spouses would want a strong connection with at least one member of the line--but wouldn't be expected to love everyone equally. No idea how they'd deal with queer spouses; Heinlein dodged around all of that.

In the long run, you'd have something between a clan and a corporation; if there's no legal limits on the size of the "marriage," you could wind up with a continent being all controlled by a single "marriage." And of course, once it gets past a certain size, the "marriage" is a matter of paperwork, and people have pair-bonds of their choice inside the larger framework, and start making social & maybe legal arrangements for managing those. (I love poly-families, but I'm aware that most of humanity is always going to pair off.)