I have two girls, and although I adore them, I'm longing for a litle boy someday. I was absolutely positive DD#2 was a girl from day one b/c of the ovulation date, so I didn't really suffer from a letdown on ultrasound day..
I am glad that my first daughter now has a sister; they are very close in that sister-y way that I never had (my sister is 13 years younger than me, my two girls are two years apart) And I plan to really make sure I've ovulated before we try again to maximize our chances of having a boy..
And it's interesting, now that I've had them, it seems that everyone I know with 2 has either all boys or all girls.. It really seems like only one in about six of the families I come across w/ two children have a boy and a girl. I wonder why that is..
It's not about not loving what you've got, I'm sure you'll love your new little guy to bits - it's the disappointment of a dream you've held close for six months, possibly longer. It's wanting someone you can share things common to your gender. Wanting some pink in your life, a daughter to connect with on a woman-to-woman level someday...that's certainly not something to be ashamed of. Nobody bashes a man for secretly wanting a son, I don't see why it should be any diffent for a woman wanting a daughter, or a son for that matter.
It's sad that people seem to need to reject our sadness and replace it with something else like "you shouldn't complain, a healthy baby is all you should want..." You're human. You want what you want. The only failing would be in not loving him, and I seriously doubt that.
I think people try to make you feel better when they say "oh, look on the bright side..." Of course there's a bright side, but you should talk through your sadness with a trusted friend who won't judge. You're entitled to mourn the loss of your dream.
FWIW, I've had two cuties w/ wonderful temperaments and I'm just crazy about them both. Part of my reason for wanting a boy was to have a child who resembled my husband. And now of course my youngest daughter is the spitting image of him, in a breathtakingly gorgeous sort of way. Their baby pictures are identical and everyone now says to us "Oh, I see you each have your own mini-me.."
I think Mother Nature wants me to give it another swing. I'm pretty sure she'll send me a screamer (girl or boy) when I'm supposed to stop.. ;) In the meantime, you'll have to tell me your secrets for having a boy.. ;)