I've seen this statement repeatedly, but little or no attempt to show why it is true.
Most civilizations have practiced infanticide to some extent, with girls always the primary victims. Ancient Greece and Rome certainly had a major sex ratio imbalance.
African and Muslim cultures, among others, practice polygamy, in which the more powerful males tie up massively disproportionate numbers of the females, leaving few for the less powerful males.
One common side effect of high male sex ratios has historically been warlike societies, in which the more powerful males aim their less powerful brethren at adjacent societies. Some of these men capture wives in the process. Others die. In either case the competition for women is reduced.
In the 1860s War of the Triple Alliance, Paraguay went from a population of 500k to about 225k, only 28k of which were men. I've read the Church temporarily allowed polygamy, although I've also seen this denied.
IOW, there is a long history of sex ratio imbalance of massive proportions for both male and female predominance. Instead of talking about how today's Chinese/Indian situation is unprecedented, I'd like to see some discussion of the precedents and what they tell us.
That was all very well when wars were conducted with masses of men in armor bearing swords.
However, nowadays, we use heat-seeking laser-guided missiles, and that sort of thing. Whether these masses of men will be of any use remains to be seen. I suppose they could invade Siberia or something like that.
The absence of men has occurred very commonly, throughout history. Men die off in wars, resulting in polygamy. The absence of women is what is rare.