I think there are two things here. Firstly, someone personally not wanting a child of a certain sex (I think this is sexism, and is somewhat morally objectionable, but I also think that bodily autonomy simply trumps other values in this case). This isn't a crime, though it certainly does give fuel to people who say feminists hate men. I daresay they would say this regardless of what we write about, however.

The second thing is calling for, attempting, or legislating in a way that allows for a systematic reduction of one sex. The latter seems relevantly similar to genocide, in my view. I do consider it a major moral outrage this this is happening to girls in some parts of the world, and I would strenuously object to any suggestions that the same thing be done to boys.