"I don't see what's wrong with asking if a baby is a boy or a girl."
It's not "wrong", but when people ask that question they tend to assume that
there is an either/ or answer (sometimes the true answer is "no"), and that
the answer says something important about who that baby will grow up to be.
In an ideal world it wouldn't, because, as you rightly point out,
"gender is mostly a social construction."
Again, I highly recommend this video as an introduction to the range of
issues involved:
https://www.youtube.com/watch?v=xJv1QWdofEE
