When I dreamed of having children, I always wanted a boy. When I found out I was pregnant, I just knew it was a girl, and was delighted, which confused me somewhat. She was indeed a girl.
With DC2 I was convinced it was a boy, and when DD2 was born, I momentarily felt disappointed, and have felt awful about it ever since. It took me a while to get around it, but obviously I would not change the little rascal for anything. She is a proper tom-boy too, whereas her sister is very girly.
With DC3 we chose to have a gender scan, which revealed a boy. Everyone had assumed we were having a third to have a boy. We just wanted a third child. Before the scan I managed to convince myself it was a girl, probably to avoid disappointment as chances were it was a girl. It was a huge shock to be told I am having a boy, and it still has not stuck in. Thankfully I have another 3 months to get my head around it.
Our emotions are complicated things, especially with all the hormones on top. It is perfectly normal to have a preference for one gender or another. What is not normal is to consider killing a baby purely because of gender, or not loving a child because they are the wrong gender.
When you see your baby it really doesn't matter what is between their legs, but it is important to not build up an image of an unborn child unless you know beyond doubt their gender. Scans give an indication, but the only way to know for sure is with invasive tests like amnio, and I would not risk the life of my child just to identify gender.