Also, it could be because of our fears of not getting a girl. The "finding out it's a boy and then being devastated" part.. It's our worst fear!
But, I am hoping it's because we got our "opposites" in there!!
With DS2 and DD I dreamed the correct gender for them both (had no gender dreams with my first). When I fell pregnant this time I began to have very vivid dreams that the baby was a boy. Because they had been right previously I assumed they would automatically be correct this time too. I had similar dreams to OP, that the nappy was being peeled back and the genital shown to me and I would feel an overwhelming disappointment (and even in my dream also feel guilty for feeling that way). My last dream I walked into a room and DH said 'I know the sex of the baby', I was excited hoping he would say girl but he said 'it's a boy', I sat down hoping not to cry and said 'are you sure?' and he said 'yes, definitely but don't worry the next one will be a girl'.
I thought that was it, definitely boy but no - according to the scan it's a girl! I still feel nervous about the dreams, like my girl will turn into a boy because of them but I also think it's like the poster above stated - my dreams were reflecting the fears I had. Since finding out the gender I've not had a single gender dream.