Wow, so many replies! Thanks to everyone, I definitely feel so much more excited already about (maybe) having a boy.
I really love the humour and honesty on here! I quote mumsnet posts on a daily basis to my husband, he must think I've joined some kind of cult.
I just wanted to say a special thanks to @rightreckoner - that's such a refreshing comment and really reassuring. There is so much pressure on us to feel the 'right' things at the 'right' time, that it's easy to forget that everyone's different, and that's fine!
Part of this pressure I think is the idea that you should be only excited about having a baby, and that there is no place at all for anything else, like a little bit of disappointment because you had dreamed for whatever reason of having the opposite sex. (By the way, @BrieAndOatcakes, that's funny, I think I know more women in RL who wanted a boy and were slightly taken aback when they had a girl. But I do agree about the unfairly negative expectations about boys in society, although they're not the things I worry about.)
I had all but given up hope of conceiving naturally and feel incredibly lucky to have narrowly avoided IVF, but that doesn't mean I don't have any wishes or dreams for my child beyond it being healthy, even though that's obviously by far the most important. That would just be unrealistic. As lots of you have pointed out, having both positive and negative emotions while you go through pregnancy is normal and it doesn't mean you're not excited (I am BEYOND excited!), or that you're not going to love your child unconditionally. I think it's healthy to be honest about these feelings so that you can deal with them, rather than feel guilty in silence. Women do that enough, surely!
Just to reassure the few people that seem maybe a little offended: I don't see the world in gender stereotypes (I'm not one, for starters) and I agree that gender is only a small factor in a child's personality. I will certainly not teach any child of mine that societal expectations based on sex/gender need to determine how they should behave, look, or feel. Whoever my son or daughter decides to be I'll love and support them, and I'm really looking forward to finding out!
It's really just my dream of magically turning into Lorelai Gilmore if only I had a daughter to play my Rory, that I can't seem to shake. 
Sorry, that turned into a bit of an essay. I'll shut up now.
Internet hug to all of you.