"Stupid people breed too much" is a proposition that is either true or false, within some worldview.

(For example, how stupid is stupid? How much breeding is to much? Is there even such a thing as too much breeding? All these are variables up for debate.)

But preventing the spread of an idea that you fear may be true, simply because you don't like the consequences, is intellectually dishonest.

Would you endorse suppressing the idea that the earth orbits the sun just because you lived in a milieu where the primacy of the church was more important than truth?

Argue against eugenics because it's unethical to prevent people from reproducing (and therefore no amount of "stupid people" reproducing is "too much"). Don't cloud your judgment by denying propositions that you fear may be true.