I'm pregnant with no1 and just found out it's probably a boy (sonographer didn't sound v sure!)
It made me realise how much my daydreams have involved a baby girl as I did feel a bit disappointed. I've been doing a bit of reading about it and apparenty it's not uncommon (google gender disappointment).
I think I'm okay with a boy now, it's just a readjustment to day dreams. For me I think my initial disappointment stems from growing up with a mother who said she was terrified of giving birth to a boy, and because the males in my family are traditionally grumpy or aggressive people. (One of the common causes is having been abused by a man in the past, and I think this is where my mum's terror has come from. It sadly meant she treated me as the favourite child and my brother got the brunt of her tempers).
I'm getting over it by reminding myself that my little boy will have his daddy's genes, and my DH is the most wonderful, caring, amazing husband. And also there is NOTHING I can do about having a boy, and I'm damned if I'm going to let my preconcieved notions about men damage my precious son. I haven't discussed this IRL with anyone other than my DH as there's no way that I want it getting back to my son in future that his mum was disappointed he wasn't a girl.
Remind yourself about all the good things that a baby means, and disassociate these things from gender e.g. I'm looking forward to the feeling of holding that tiny warm body in my arms, the first smile, the baby grip.
Hope you come to terms with it soon and don't feel you're alone in this. It's probably good to talk it out with someone in confidence so you don't fester on negative feelings. And be confident in the fact that the vast majority of people appear to recover from this as soon as they hold their baby!
Good luck x