What is there to understand? It's an emotional reaction. I don't understand why some people hate to wear yellow, or are disappointed in receiving a gift or are scared of spiders. Doesn't mean I feel the need to demand an explanation.
You sound quite naive to be honest. Of course the are people disappointed in their child. I've been nothing but a disappointment to my parents. I don't follow their ideals i.e. do everything they how tell me to, i let my children watch television, let them decide how to dress, let them talk at the meal table etc. I chose what to study at university (still not forgiven, my parents used to tell people I was studying something related to my sisters degree) and didn't follow my siblings into the same job field. I'm the one family member who is never mentioned in the Christmas letter. I am always compared to my sister and found wanting.
And yes, I was horrified that I would have a DD. And yes, I cried and managed to stammer "at least she's healthy" at the scan much to the amusement of DH and the Dr. I feared MIL (no girls in the family for a couple of generations), that she would be like me, that I wouldn't know what to do with her hair if she wants long hair when she's older, that I would be judged for dressing her in trousers and tshirt rather than an outfit. Once she arrived, I got used to the idea. MIL showed no interest in her whatsoever, she chooses her own clothes every day and is far easier than DS ever was! I feel guilty for hating every second of the pregnancy. I also spent every second of the pregnancy petrified that I would lose her as it would serve me right for for not wanting to have a DD.
Fortunately I have friends who don't judge me for this (at least not to my face), who give me advice on girly things I have no clue about and pass on lots of clothes for her, and don't feel the need to make me feel (more) guilty with their patronising "oh, I don't understand how anyone could feel like that" self righteousness.