The trouble with this reasoning is that universally accepted norms cannot be based on lines drawn at arbitrary points. There must be a strong focal (Schelling) point where the lines are drawn, or otherwise they will soon be pushed in one direction or another. Or to put it differently, slippery slope arguments usually have at least some validity.

With this in mind, arguments against things based on placing them into common categories with bad things can be perfectly valid in a very important sense. Yes, they usually produce powerful propagandistic rhetoric as a side-effect, and sometimes that is their primary purpose. However, often there are no convenient focal points where you would ideally like to draw the lines, and the best available focal points are around some much broader category, so if you won't draw the lines around the broader category, there is a very real possibility that some pressure will push them all the way to things you'd definitely want to prevent.

So, for example, people who want to draw the line so as to condemn all eugenics (under some coherent and widely accepted definition of the term) have a workable focal point to defend. In contrast, if you'd like to draw the lines around some forms of eugenics and not others according to some sophisticated ethical analysis, chances are such norms would be unstable and the lines would soon be moved in one direction or another.