The ancient celtic/wiccan rituals of handfasting (engagement) & subsequent ring binding were pre-medieval, and adopted by faction of early Catholism in Britain.
So we are talking about an old Celtic/Pagan ritual anyway, in which part of the contract was the act of sexual intercourse, and a strict year and a day handfasting (engagement). Even back then, the ritual differed between different celts and gaelic communities. Pagan were not as strict, so the rules were not so hard and fast (some Gaelic cultures having 10 degrees of marriage type). Polygamy was widespread, and homosexual unions occurred, but were not considered marriage (as marriage back then was focused on the protection and rights for children, and they could not bare any). (it wasnt a good life for children not bound to parents back then, and family lines and inheritance were very very important.)
So the definition, ritual & customs obviously change with the times, vary between cultures, etc. Each generation will refine it, like our current generation is doing.
I expect in another generation or three it will change again when someone wants to marry a cyber-organism
Change is inevitable, roll with it