Nerudals that's the problem I have with it.
People choosing the sex of their baby, or even having a strong preference to choose if they could, simply reinforces this myth that there is a big difference between boys and girls and that if you have a girl you will get experience X (pretty dresses, dolls, colouring) and if you have a boy you will get experience Y (football, trains, rough and tumble).
So it perpetuates the stereotypes and expectations for each sex, and that is harmful in all sorts of ways.
It would be much better if people recognised that sex alone doesn't tell you a single thing about what kind of child and what kind of parenting experience you will have, other than how they will pee.
There's also the slippery slope issue. Right now, genetic testing can tell us whether an embryo is male or female, and test for certain health conditions. What about if testing could, in the not too distant future, discover whether a child would be tall or short? Have blue eyes or brown? Light skin or dark skin? Be clever or not so clever? Would it be ok to choose the more preferred genes and discard the others?