Congratulations xx
We are a couple of weeks off of our 20week scan, it's our first pregnancy. We aren't going to find out (unless I suppose our baby flashes us on screen and it's obvious). My DH wanted to know and has a slight preference for a boy. I don't mind really (though I'd like both genders eventually) but I didn't want to find out. He doesn't want to force me to know, and doesn't think it would be fair for him to know and me not to. So we'll wait.
My reasons against it: Part of it is wanting a nice moment at birth when DH tells me whether we have a son or a daughter. Part of it is that I don't want to have different expectations based on gender and the easiest way to avoid them during pregnancy is not to know! I just feel there is enough of that gender-specific nonsense once they're born. Part of it is to avoid being overloaded with pink/blue items from relatives.
Hmm.. wonder if I can have my partner tell others the gender after birth by saying 'We have a healthy girl/boy but she/he doesn't necessarily like pink/blue'. 
Speaking as a girl who wanted her room painted blue and was refused because it was a 'boy' colour. Course, knowing my luck I'll have a pink princess who wants their room painted pink - whether they are a girl or a boy!
My room ended up yellow when I was little, and later was green and dinosaur wallpaper. I now share a BLUE room with my DH, success at last 
I won't deny my kids pink btw, I just don't want them to be defined by the toy marketers. I wouldn't go so far as refusing to tell people the baby's sex, though I understand why some people have, but I think it draws attention to the issue. My point is that girls and boys can have whatever interests they like, not that they shouldn't be girls or boys.
Not having a go at those who do find out the sex, or those who enjoy the whole 'Team Pink' deal. It's just not my 