Slightly ot but there is absolutely nothing wrong with finding out anymore than there is anything wrong with waiting. I do find it patronising on these threads where anybody tries to justify their way as the better way. You find out or you don't. You're not wrong or impatient or any other thing. You do things your own way. It's utterly unlike sneaking a look at Christmas presents. For some people it's merely a statement of fact. In the same way that you idly read books/emails/forums which tell you 'your baby is currently the size of a plum', or 'the baby has fingernails', some people are curious to find out what sex the baby is. Purely because they can. It's feck all to do with gender stereotyping for me too. If you believe gender stereotyping is that all pervasive (and I agree in some ways it sadly is) then you're going to fight that battle anyway. If YOU don't plan to stereotype then it matters not when you find out, your child is still your child, you just happen to know what's between its legs.
And the op and her dp are having the very problem reflected by this thread. If you want to do it one way, you believe that way to be better than the other way and are in the business of convincing other people you are 'right'. But as it's not theoretical and you do have to make the decision, I'd suggest you have a very frank conversation about him respecting your ability to make a decision for a start. It sounds like he's bullying you.