Unnatural Selection

Unnatural Selection: Choosing Boys Over Girls, and the Consequences of a World Full of Men, by Mara Hvistendahl is a book about the sex selection problem, particularly in Asia. I ran into this book via Tyler Cowen, who has a good excerpt from the book. Here's a review on the Amazon.com page:

“Unnatural Selection is an important book and a fascinating read. Mara Hvistendahl is a delightful writer: witty, engaging, and acute. But the tale she tells is deeply disturbing. Asia alone is missing 160 million women and girls, a number equal to the entire female population of the United States. According to Hvistendahl, the culprit is less deeply rooted cultural gender bias than rising wealth, elite attitudes, and Western influence and technology. Development, at least for the coming decades, will produce not only fewer children overall, but also many fewer girls. The result is a future for many parts of the world, from India to China, Azerbaijan to Albania, where brides are much more likely to be bought, women are much more likely to be trafficked, and men are much more likely to be frustrated. For the present, women who are pro-choice must confront the stark reality that the availability of ultrasound and ready abortion are sharply reducing the number of women in the world.” —Anne-Marie Slaughter, Bert G. Kerstetter University Professor of Politics and International Affairs, Princeton University

So how about it? If abortion is acceptable, and we can identify the sex of a fetus, what's the moral problem with aborting based on gender bias?

Or any other bias, for that matter?