Robin's Celta world is populated with descendents of (I think she said) twenty-five noble families who fled from Earth in a space ark to escape persecution because of their psychic abilities. Several generations aboard a space ark concentrated their psychic abilities. Twenty five families is good -- just about the right amount of inbreeding. There were 74 men and 28 women on the Mayflower.
One of the most creative novellas, in respect to breeding and future twists in our current taboos, is Ravyn Wilde's A.D. 2203: Adam & Eve (published by Ellora's Cave). Ravyn's premise was that humans would live in harmony with shapeshifters, vampires etc. However, since werewolves have special breeding needs and only fall in love permanently with one life mate, it is the law of the world that-- if a woman turns on a werewolf with her pheromonal scent while she is ovulating and he manages to find her and bite her and have sex with her-- she is legally bound to marry him.
Now, there's a twist on sexual affirmative action!
In Brave New World, humans were divided into 5 castes according to intellect, from Alphas to Deltas as I recall (it's been over 30 years). Alphas bred with alphas and rules the world. Deltas bred with deltas and did the dirty jobs.
In Mary Doria Russell's alien world of "The Sparrow" breeding rights went to the first-born in the family. Third sons had to be satisfied with sterile unions either with genetically incompatible other species or with homosexual partners.... unless they distinguished themselves.
Mostly in science fiction romance, authors don't stray too far from our current sexual mores and taboos. We avoid or gloss over bestiality, and incest, but homosexuality is as acceptable now as it was in the Greek heroic days of Achilles and Patrocles.
Over the course of various human histories, there have been many laws, taboos, prohibitions and social conventions restricting a person's choice of with whom he --and especially she-- may marry or breed. Most have been "Thou shalt not" type laws, rather than "Thou shalt."
I wonder whether the pendulum will swing. When we were discussing one-world government, most correspondents envisaged a confederation of separate, harmonious states presumably (although this was not explicitly stated) along existing racial or national lines.
But what if the "Melting Pot" idea became formulated into law?
Conversely, what if humans followed the Brave New World model, and we were encouraged to become specialized, like ants. Teachers would breed with teachers to produce super-teachers. Warriors would breed with warriors; geeks with geeks; actors with actors.
That's not too far fetched, is it? In fact, throughout history --until modern times-- royals married royals.