It's about consequences of what you say.

If you say you like eugenics, the promotional consequences of your words are not limited to some nice, non coercive, purely voluntary eugenics. Instead, it is more or less spread over eugenics as frequently understood.

If you want to promote the voluntary measures, you say that you are pro choice, pro birth control, and believe that parents should have the right to improve health of their future children. If you want to also promote the involuntary variety, that's when you would say "I like eugenics".