I can likewise imagine (and could prefer) a scientific civilization that is freely polyamorous, atheistic, Eugenicist, etc.

Scientific civilization that actually understands science of biology would steer clear of eugenics.

For pure pragmatic reasons - breeding better (whatewer value of "better" you choose) humans would last at least several centuries - and the problem is that you do not know what traits would be needed then.

Here is one actual historical example of human breeding. Had Frederick II and other kings of Prussia continued the work, Germany could well have a race of eight foot tall soldiers - just in time for WWI.