Personally for my first child I was hopeful for a little girl, largely because I figured I'd know what to do with a girl more (eg. How to talk to her about puberty once she reached that age). However I ended up with a boy, and then I think when it hit me how terrifying the world felt as a parent, so for my second pregnancy I switched to wanting only boys because that seemed slightly less scary.
I'd imagine as well that wondering how you can raise a boy and somehow avoid him taking on misogynistic views and behaviours is going to play some part on a board like MN, it scares me that one day my sweet gentle boy might end up taking on board those messages that society bombards boys with.
Essentially the world is currently such a misogynistic place that it makes perfect sense to be worried about having a child of either sex.
Plus you need to remember that not all MN posters would identify as feminists - let alone rad or GC feminists, especially outside FWR, so a fair portion will want a girl because they think they'll have more in common with a girl (eg. Stereotypes like clothes shopping). And a lot will have grown up playing with baby dolls which are largely female, which will make it easier to imagine babies as girls. And of course there is the idea that girls are more likely to stay close to parents as they grow up, be more helpful around the house, and take on caring responsibilities.
I'm surprised anyone would be surprised that women tended to want daughters not sons.