Forced eugenics is obviously unethical but whats so bad about selective breeding in general as long as your going for intelligence, athleticism, musicality etc. and not country of origin.
If you breed for muscularity and athleticism everyone is physically fit but you no longer have smart people that spend most of their time studying. Too much intellect and you suddenly are in shortage of people that spend most of their time getting fit like firefighters and soldiers.
Ethically, there is this concept of self determination for humans, that our existence especially with flaws is what gives us individual traits. Previous generations should not amplify any trait on purpose because that implies we are bread for a purpose. We are not purpose built machines or animals, we get to decide what purpose suits us best which is individual self determination.
There is a difference though between a person finding a mate because they find a feature attractive (reproductive selection) and selecting a mate because that mate will amplify that feature in their offspring.