Although I understand the perils of technology of this nature, I cant say that it will pose a problem in the big picture. Not if it stays an
optional, expensive, elitist procedure.
You mentioned that its not allowed in some other countries and its an expensive process. This alone would keep the practice in check. The US only
represents a small fraction of the worlds population. And the amount of people who actually have this available to them im sure is hardly measurable
against the worlds population. I.e. the impact could hardly be measured.
Now, if this technology became legal and available to the entire world, then things would become interesting to say the least. Natural selection and
evolution has made human reproduction the way it is for a reason and I agree that the ability to choose a gender would alter that in ways we could
never predict.
In the example you stated I dont see any issues. After all, they didnt resort to this technique until they already had three girls. As far as having
twins is concerned, thats a by product to fertility help, not gender selection.
This is definatly something that needs to be watched very closely.

