genetic purpose
Our genetic "purpose" could also be viewed as to also dominate, kill, conquer, enslave, eat and anything else we can get away with. Some day robots will be offended by your short sightedness? Homosexuality also doesn't genetically do much for procreation either. YET. Doctors may fix that.
Maybe in the future, robot-babes, robo-wombs, will clone humans/robots from your DNA mixed with the DNA of the robo-partners creator.
I'm guessing the next step for marriage is poligamy though? At least they can make babies. Unless it's homo-poligamy? or robo-poligamy, before they invent robo-wombs. What about alien marriage? or Gentically modified apes with IQ's of 180?
I think an easy litmus test might be if it can make a baby, they can call it a marriage? So no homos yet, but in like 5 years it'll be ok thanks to stem cell research?