I think people just aren't sure what to say and being positive about it is better than being negative. I never know what to say when I know someone's gotten the gender they didn't want. I know there's no good answer, from a personal standpoint. The best thing I've ever heard is, "Oh, that will be so wonderful for your son. He'll love having a brother!" I am NOT in the GD closet. Everyone and their mom knew I wanted a girl, and I get very defensive when I exasperatedly tell people this one is another boy. I always follow quickly with, "So, there's no way this is the last one since I just can't handle being the only girl in the house." It has to be awkward, especially coming from someone with no kids or a PP. I try to be nice since I know that there are just no good answers. As long as the response I get isn't bitchy or mean, I try to just nod and be gracious for the congrats...and remind them that I'm not giving up yet!
Crunchy mama two DS and married 6 years to my wonderful DH. I'm NEVER having another boy!