When I think of an ideal society in the future, it is one that has the greatest standards of living for every individual.
Greatest standards of living equates to greatest personal freedoms without harming others.
With this in mind, polyandry and polygamy, when consensual, should be legal.
In addition, if science progresses enough to eliminate the birth defects associated with incest, then it should also be legal, no matter how disgusting we might find it.
And without wishing to be too controversial (lol who am I kidding) some would say that allowing homosexuals to raise kids also screws them up for life...
I agree. But this is because of society's inability to accept such families. They will get the shit kicked out of them in school.