I'm not sure why directly selecting for valued traits is worse than indirectly doing so though the choice of a mate.
Because it is dehumanizing and commoditizes human life. It involves children being treated as consumable products valued only for the benefit they confer to a third party, rather than respecting their innate value as humans.
If I had the chance to de-select the life threatening allergies my son has, I wouldn't have hesitated for a second. Looking back, there is nothing else I want to change now. They each have their strengths and their faults, and that is what makes them who they are.
But if I had the power to make other changes before birth, could I have resisted crafting 'perfect' children? I don't know.
