The main issue if you want to be completely amoral about it (Note, not immoral) is what to do with excess male population when war and accidents, hunting, getting trampled by oxen or whatever "burned off" excess males isn't going on because of relatively stable and safe modern lifestyles.
Some of the polygamist cults in the American West are known for shunning or expelling young men for manufactured wrongdoings.
Not even a result of polygamy, just infanticide, differing standards of care, and sex-selective abortion, we'll see what the impact of 40 million excess males (and growing every day) in China, and in other Asian nations to lesser degrees will be.