Despite the fact that gender disappointment is surrounded by a lot of silence, it is not all that unusual, and not something you need to feel guilty about. It can happen to any pregnant woman, but some of the more common reasons for gender disappointment is already having several children of one gender, having experienced childhood sexual abuse, or simply feeling unable to connect to a baby of a certain gender, whether that is a boy or a girl.
I experienced gender disappointment with my second child myself. I already had a beautiful daughter, there were no boys in my extended family at all, and I could not imagine being the mother of a boy. Of course, I felt guilty and stupid about my feelings, but they existed nonetheless. Once I got over the fact that my little boy had male reproductive organs, I realized that he was the most gorgeous, sweet, wonderful little baby and that he has perfect penis and all! That is the one thing to keep in mind if you are having a boy, and you wanted a girl or a girl, and you wanted a boy.
Babies are unique human beings, and their sex doesn't really have a lot to do with the love you will feel for them after birth, even if it feels different right now. Everything will be just fine, and you will love your little one like nothing else in the world! If you are still trying to conceive, you might want to read about gender selection methods. How to conceive the gender of your choice, and natural ways to have a daughter might be of interest to you.