GhostWriter16
Deity
Polygamy I think. Less icky than incest so will enjoy more support, and also fully consensual.
Incest can be consensual as well. Though remember, polygamy is a key reason Soc Cons use to justify banning gay marriage.
Personally, I see no reason in theory such a relationship (Polygamous) should be illegal, so it should be allowed, but with the current tax system I don't think it should be recognized.
Incest I'm not sure about what point should be allowed. We should allow the experts to judge based off chance of fetal defect, and it should be banned at such a point that it would be specifically dangerous (Probably should be allowed after 55 though, then there's no risk.)