As @roomba said, I have known people say it's a 'shame' when they have another boy, (after having one or two already,) but cheer and celebrate when it's a girl.. even if the couple already have 1 or 2 girls.
I also know several women who cried for weeks at their 2nd or 3rd child being a boy, as they so desperately wanted a girl. Some of them got over it, and moved on, and treated their son with love and respect, but a few of them never recovered from the disappointment, and never really loved their boys(s.)
I have 2 girls, and not once has anyone ever said 'oh you must be disappointed to have not had a boy.' It's never happened. Yet, a number of my friends and colleagues who have more than one boy, have regularly had the 'awwww, what a shame' attitude from people who think it's such a shame they never had a girl.
I don't know why this is, but it is true that many people (women especially) want a daughter. (Even if people choose to deny this.)
As an aside, I know a few mothers who had one son ONLY, who are quite close to them, and seem happy with their boy. But when they have more than one boy, they don't seem hugely close to either one. You also get very bitchy comments about girls from some mothers with 2 or more boys.
It's a defence mechanism; they are badmouthing girls so people will think they don't like them, when in reality, they would have given their right arm to have had a daughter. I know this, because some mothers of boys that I have known have bitched about girls, and then later on, it has transpired that they secretly longed for a girl.....