I think it's okay to ask if someone is hoping for the opposite sex to the DC they already have, I think it's a natural thing to ask, however I utterly resent the implication that I'd have been disappointed with another boy!
I had 2 boys when I was pregnant with DD, and yes, I was rather hoping for a girl, because I'd love to have a girl... BUT I would never have been disappointed if it were a boy! I'd have loved him just the same.
We did discuss the fact that if I were carrying a boy, we might try one more time for a girl, but we'd have no more than 4 children regardless. When I was actually pregnant and so ill with her, I told DH that even if this one was a boy, I didn't want to try again. I was happy with 3 children no matter what they were, I didn't want to be pregnant again! (Different story now DD is nearly 3, and I'm starting to get very broody)
I now have 2 boys and 1 girl and to me that's perfect, I'd never presume to think that that is perfect for every one.
I know a mum who really only wanted boys (fortunately for her that's all she got)
My sister only has 1 boy, and doesn't intend on having any more children at all, I couldn't do that myself, only have 1, but I'd never presume to tell her that she must have another one just because it's what I would do.
People keep asking me when I'm having another, and telling me that I'm wrong when I say we are not planning any more. I'm happy with 3.
I often get this look >> 
I AM happy with 3, yes if we were to fall pregnant unplanned, I'd keep it. But failing an unplanned pregnancy I'm sticking with three. Yes, I'm maternal, yes I'm broody, yes I'm great with kids/natural mother etc. But I have 3 of my own... Is that not enough already? LOL
*And just for the record, in my own experience, my girl is most definitely harder work than my boys were at the same age! I couldn't possibly say this is the case for everyone, but for me it is so so true! Haha