Don't be ridiculous. Of course people can and will be disappointed. They generally do, after a while, love the baby completely anyway.
Sometimes gender disappointment is a manifestation of pre/post natal depression, and people should be able to express that.
Telling them 'oh, some people would kill for the child you have' is not helpful.
In my case, child 3 being a girl totally buggered up our bedroom allocations. It would have been much easier had she been a boy, but we're hardly going to hold it against her
.
Many people have issues from their pasts which will affect how they view children of a particular sex, at least until they might come to be able to love them as their own child (if they are lucky and supported).
People are complex, and easily scarred. Judging them without at least trying to understand is a horrible thing to do.