my baby's in the white fluffy clouds

If you assume many/most people will want to be in heterosexual monogamous relationships when they grow up (admittedly, this might be a questionable assumption), and if parents all tend to pick one gender, it can become difficult for people to find partners when they grow up. Then again, parents who knowingly create this problem for their children are, well, not really helping their children out, and one might hope that they'd be intelligent enough to not do that, but ISTR that China's one child policy and selective abortions have led to a more skewed gender ratio than is found elsewhere in the world.