And serious GD reflects what MsJupiter is talking about - it reflects the wider world, and negative experiences with men, where women are oppressed by men.
Traditionally the reason women were disappointed not to have girls was because they wouldn't get any help in the home and would have another person who would grow up into someone they had to serve and be subservient to. In my MIL's family, the first two of five were girls and by the time they were 7 or 8, they always had to a) get up and give seats to their brothers; b) take smaller servings of food than their brothers, though they were older and bigger than they were and c) make their beds and launder their clothes. There was also a rule that if a male was speaking they had to listen and not question his judgement.
When women's grannies say "ah I bet you'd have loved a little lady", they are often speaking from a place where a woman secretly hoped for a girl to be her companion in what was often a lonely experience. This sort of thing filters from mother to daughter over generations, and the fact that (thankfully) these things are changing doesn't catch up for a bit.
I think when people get all head-shaking/tut tut/it's all about pink sparkly shit, it sort of minimises the fact that despite the fact we've come so far, there are still significant differences in how girls and boys are socialised that are really uncontrollable once your child leaves the home and peers become a greater influence. I frequently look at what my boys are expected to be in terms of their socialisation and how they are expected to express emotion or adopt some sort of bullshit macho persona etc and it pisses me off. My five year old will run up to a peer on the playground and hug him or kiss him because that's what he's used to at home and get pushed off like he were vermin. This stuff hasn't actually vanished from life, and it upsets me that it is so. It doesn't surprise me that women who have experienced severe oppression in their homes - including violence and abuse - feel highly ambivalent when they are going to have little boys.