I actually agree with you on that. Eugenics, genetic modification of embryos, transhumanism, radical social policies, and many related topics are likewise taboo, and most people wouldn't dare to even mention such ideas in polite company.
To use eugenics as an example. Those who speak about it always, without exception end up isolating a single group that they believe is inferior. These people often hold adjacent beliefs that make the lives of other humans significantly harder, to say the least.
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".