I went through a period of exploring the history of marriage because we are often sold this idea of "traditional" marriage, but really our ideas of what is so called traditional are just the Victorian ideal of marriage. If you really look at the history of marriage, there were many arrangements which nowadays would sound unconventional which were popular at different periods for various economic and social reasons. For example, in Venice there was a period after the black plague had hit when men would marry quite young women - teens really - who were often to play the mother role to children her age or older from the husband's first marriage after the first wife had died from plague. Or in England there was a period where most working class people wouldn't marry until they were in their 30s and they already had a toddler or two, because they would wait until they had some financial independence before tying the knot. So, I often question these linear interpretations of marriage as evolving because I think they tend to look back only a hundred years or so, and if you cast a wider net then you get a lot more interesting ideas of what marriage has been and can be.