What Sara said.
If it is because of a genetic condition that is life threatening/not compatible with life and that genetic condition will only be passed along to one gender then I'm ok with it. If it's just because you want a child of a particular gender then I really think that you need to deal with your gender disappointment and move on.
I get that some cultures value sons more but I'm also of the opinion that some cultural traditions are just plain stupid and son worship is one of them. It is damaging to sons as well as to daughters (I say this as the wife of a victim of son worship).
I don't think that people need to be concerned about balancing their families - my family is not unbalanced because we have two sons. We're unbalanced because we have issewes The way I see it I have 2 sons and my brother has 2 daughters - the balance is perfect once I look outside my nuclear unit.