I'm a mum of 2 boys - and yes, I have noticed the trend you talk about.
It's been said time and time again, but this is why I don't think people should find out the sex before birth as it's very hard to be disappointed in a real baby.
You have nine months to fantasise about your children and their future, and inevitably have pictures in your mind re: what you child is going to be like. I think , purely because we're women, it's easier to picture a girl- a mini-me - and often , without consciously realising it, many women get fixated on that picture. When they hear it's a boy at 20 weeks, that image is shattered and they don't, at that point, have the real live cuddly baby to make up for it.
I didn't find out either time. With my first, I wasn't at all disappointed- I was so delighted to have a healthy baby, and so excited to be a mum for the first time.
With my second, I had thought he was a girl, and did feel a bit flat when boy no 2 first appeared. However, I can honestly say that these feelings diminished very rapidly, and,by the time he reached the really cute, squidgy baby stage, 4-5 months, I was smitten. A few months' on, I am completely gooey for him- many comment on it!- and I feel awful that I was ever disappointed.
All of this is really to illustrate that, if you do find yourself feeling disappointed, don't worry, these feelings are bound to fade and, a few months' down the line, disappear completely.
Also, I think gender is something mums fixate over in the early years, when the babies aren't doing much, and there's not much else to talk about, but, as they grow up and develop personalities, and there are so many other issues to think about, gender- as a conversation topic-fades into the background.
Also, please remember that the negative male stereotypes being bandied around only apply to a handful of children. It never ceases to amaze me that just because the media tells us that SOME boys are doing worse at school and SOME boys are more physically aggressive that girls, mums seem to think that ALL boys are like this.
FWIW my oldest son(3) is gentle, considerate and calm. He has never been aggressive, he is very friendly, and much less manic than many of the other 3 year olds I see around. Also, many of his (mostly)male friends are very similar. So I am constantly surrounded by many delightful examples of small boyhood! I sometimes feel I would like to invite nervous mums expecting boys to watch them all playing - I think it would make them feel better.
I don't wish to sound smug- for all I know, my son's personality may change completely further on down the line. Also, I don't want to attribute my son's loveliness to anything I've done - I just wanted to point out that there are many, many lovely boys out there!