Marriage itself seems like an odd institution to me. It's been a lot of things, many of which aren't relevant anymore.
Since inheritance does not need to be established by legal marriage, it doesn't really serve that purpose
As a religious and social right and a cultural tradition, it still has as much validity as it ever did, but that doesn't mean it deserves its own special legal thing
I dunno, it's convenient for taxes, insurance, immigration and guardianship of children, but otherwise seems like a rather outmoded thing to have written into law. So far as all the things I just listed, I think it would be only mildly more difficult to deal with without marriage as a legal institution.