> People do not fight for the protection of reproductive resources. The peaceful way birth rates fell around the world these days is proof of that.

People have fought for a number of reasons, material resources AND reproductive resources are usually key to those. Historically, raiding for woman and resources has been a constant. A big tribe is a safe tribe, more women is key. Even in modern times, you can see the pressure in expensionist/aggressive societies for women to have more children.