The historic basis of relationships was for procreation and child rearing purposes. In the future I expect that to not be the case. either with designer-babies, or just plenty of non-natural birthing solutions as to make the next generation make-able without needing to go through a regular-family structure.
How is this relevant? All these technologies are for producing embryos. You still need people to raise the children the same as before. And I would be very surprised if child-raising AI isn't sex-bot complete (ie if we didn't thoroughly decouple sex from human relationships long before we decouple child rearing from human relationships.
Raising children is definitely a factor in "why we have relationships", but for now I was talking about "why we have taboos around relationships that happen between close genetic people", especially when we solve the problem of close-genetic negative effects.
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