You are correct that I was conflating the two subjects and I appreciate your sticking to the one that matters.
I think what you're not pointing out in your example of slavery as an "affront" is that slaves were forced into slavery against their will. Of course that is wrong and nobody's arguing for something like slavery to come back, although some would probably not mind that -- and, in a tangent, I could argue that we're all still slaves of just a different sort.
In the end, I suppose it comes down to where "liberty" falls in your value hierarchy. If you place it above all else, I'm sure what I'm saying must seem like madness.
I guess that's the crust of the argument. I'm originally from Iran and when I moved to the U.S., I saw that the individual liberties afforded to us (or rather not taken away) are unparalleled in the rest of the world (Yes, including countries like Norway and Sweden). Because of that, I agree with the OP's "slippery slope" argument. What's a slippery slope to one is not necessarily so to others. Even what's a slippery slope to most is not grounds for laws against or banning it. Case in point, take the treatment of gays throughout the ages. If you took a poll in the 50s on whether homosexual relationships should be legal, you would have gotten an overwhelmingly "No!" answer. Just because most people don't like something, it doesn't mean that it's wrong. If you ban someone killing themselves and filming it while doing so under the guise of it being an affront to human dignity, what's next? Why should prostitution be illegal? Should mutilating your body be illegal? Do note that, in my examples, I am only taking this stance when it involves adult humans wills (the term adult to be defined by local laws, I guess).