Not that incest is a good thing imo, but technically so long as couples don't have kids with each other, I don't really see why it's any more controversial than homosexual couples, provided that everyone makes the decision to be together consensually after the age of majority.
This is assuming that children would be conceived with non genetically related partners. That said, I really feel like incest is a thing that would hold people back from developing as individuals, as the process of going outside your family to find yourself (as well as a mate that fits you) isn't just good genetically, but also for personal and societal growth, imo. It adds the power of diversity not only to genetics, but ideas and traditions.
Basically, heterosis (hybrid vigor) but applied to culture in addition to biology.