From what I've read, increased abortion and infanticide rates for females due to a cultural preference for males was the primary cause of the gender imbalance, not the one-child policy. However，the one-child policy may have other negative consequences.

I thought this just caused any young women to be undocumented rather than what is assumed from the statement "one child policy" (i.e. pregnancy termination)

Western countries don't have the incentive to select for men to such a perilous degree. Chinese society is structured so that children take care of their parents in their old age, and men have more earning power to do so. Men have higher-earning careers over here, but the parent's aren't quite so dependent on that.

There's a very strong personal liberty counterargument, but then again overpopulation is dire and density itself also puts pressure on personal liberty.