Oh Naisy, try not to be too hard on yourself, your emotions are heightened by your hormones at the moment. People on here have cried about all sorts of silly things because of hormones - and you are genuinely a bit disappointed, so don't feel bad about being a bit sad about it.
We decided not to find out the sex (am now 30 weeks). One of my reasons (besides being happy with a surprise 
) is that gender is only a tiny part of who our new child is. The world isn't seperated into two personalities based on gender (boy or girl). There a huge variety of things your child could be: cheerful, loving, loyal, energetic, quiet, determined, generous, gentle, bold, adventurous, sensitive... etc.
You are currently upset because your idea of what a baby girl is was a bit brighter in your mind than your idea of what a baby boy is. Its understandable that you have placed so much emphasis on gender, much of the excitement our society has around pregnancy and newborns is about the baby's gender. But that's only because its one of the few things you find out about a baby at birth or during pregnancy. It isn't really important. Those other things, those personality traits, are more interesting and fun to discover over the years than whether a baby has a penis or a vulva.
If it is still bothering you after the initial shock, then it might be useful to have a think about why having a boy is so different to having a girl in your mind. Are there things you wanted to do with a girl that you don't think a boy would be interested in? Because when you start to look at that stuff, you might realise that there is no guarantee that a girl would have been interested in them anyway, and nothing to rule out a boy being enthused by your interests either.
There are very few absolutes. Yes, you won't be able to hold your son's hand as he personally gives birth, but your daughter might not have had children. You won't have the exact same talk at puberty with a boy as you would have had with a girl, but he can still help you bake, might still enjoy cuddles, can still play with dolls. He could still grow up to be a primary school teacher or a nurse. Few things are determined by gender. It'll be down to his personality, and he'll only write off some things as 'for girls' if you and those around him have that expectation.
Read some feminist stuff... it'll make you angry about how much emphasis is put on gender..
As a girl who hated pink and ended up with dinosaur wallpaper for most of my childhood... I'm rather opinionated about this stuff. (My sister is more classically girly and into pink and clothes, so it is just personality).
Of course we all need reminding sometimes that the 20week anomoly scan isn't just an exciting chance to see our babies and maybe find out his/her gender, others have sadder experiences. Our sonographer was friendly, but very business-like about checking everything she needed to check for, we were glad that she was through and she was happy with the baby's health. If you are upset by your sonographer's manner, maybe you could put in a complaint? But someone else suggested a private scan for you to have a more gentle bonding experience now you know you have a healthy boy, if that appeals to you, go for it.
Blame it on hormones, there are some advantages to being pregnant.