This seems like a slippery slope argument, much like those who claimed gay marriage would inevitably lead to polygamy and thus should be banned for fear of making it easier to normalize polygamy in the future.
I'm having trouble understanding where this slippery slope is coming from. It's one thing if you were talking about assisted suicide, I can see the argument there, but eugenics is something that is forced upon the population. And no, having to watch two people of the same gender kiss on TV is not forced.
An extremely large proportion of the US population would not agree with this if you include polygamy in "whatever non traditional arrangements they want".