Min, You are very right about sex being near impossible to predict as early as 12-16 weeks. Even when babies are physically examined as early as 12-16 weeks it can be difficult to definitively tell what the sex is. When I had my Angel baby at 15 weeks the doctor was very sure it was a boy but still wouldn't be definite until the pathology came back. He was a boy but as I said these little ones are so teeny tiny...
Technology now is so very clever and with a highly experienced and trained ultrasonographer the 20week u/s is a pretty safe bet.
Telling a family the sex of their baby at an earlier u/s I believe would leave the family wide open to much confusion such as that that Bellabub is experiencing. It makes me cross to hear these things happening - I think these people need to know they have peoples emotions and dreams in their hands and they have a responsibility that comes with that.