I pretty much fully support designer babies as a concept, I'm only against clearly abusive and harmful actions relating to it.
... You don't see the concept itself as abusive? Not only would it result in the upper classes (those who can afford such screening/treatments) to have a
genetic leg up on the lower classes, you also run the very real risk of parents engaging in very prejudicial behavior in selecting which traits their children have. Parents are not geneticists, do not understand the issues involved, and should not be allowed to choose the genetics of their children, considering the potentially very difficult to solve ramifications.
"At least not in a negative manner." The aren't choosing a sex based on the belief that one sex is inherently inferior, just for the sex itself. It's similar to choosing an actress: you aren't expected to hire a guy to play the role of a girl due to gender equality. They need that specific sex simply for the sex.
You don't have to believe one sex is inferior to the other to be sexist, and expecting
anyone to subscribe to specified roles based solely on what private parts are attached to them is
going to have starkly negative consequences. There is no such thing as strictly positive or even neutral sexism.
Truth be told, I'm on the fence about that. Being an adopted, unwanted pregnancy, I've put a lot of thought into it. That idea of not existing just because a person is inconvenienced or burdened is actually quite frightening.
Allow me to draw an analogy: You could say exactly the same thing about birth control or anything else that prevents conception/childbirth. The idea of not existing because my dad decided to wear a condom is pretty frightening to, but only on a personal existential level and not in a way that should actually affect my opinions about anything.