You're taking that quote out of context. The point is not what could happen today, but what could happen prehistoric groups. If you have two prehistoric tribes in conflict with each other it's entirely reasonable that you'll lose half of your combatants over a period of years. If you're losing men then you can replenish your numbers just as effectively, if you're losing women your capacity will be compromised.
In more recent history, during WW II the Soviet Union had an army with 34,401,807 men. The official tally was 8,668,400 dead from various causes. Assuming that this military included all men of draftable age (which is basically correct), about a quarter of men in that generation died. Yet since those losses were born mostly among men (the Soviet Union lost many civilians as well, it was a bloody war), the reproductive capacity of the society was not impacted nearly as badly as their population was.
Given that we have something like 7 billion people, the odds of losing half of either gender are fairly low. If it's because of something nuclear -- you will probably want to be on the casualty list, not the survivor's.
I wish I could downvote this post. It's flamebait, and it's irrelevant. Agree with the commenter who wondered why "debates" like this go on amongst people > age 13.