Why are women treated so badly?

[cally]

So, parents sell their young female children into slavery. Slavery is endemic throughout the world and female slaves/sex slavery is increasing. In India and China and elsewhere there are not enough females to marry because the female babies are killed. Routinely, women are raped and abused during war.

Women are routinely lesser than men. I don't know why. There is no reason why humans should prefer males over females. It is a cultural, learned, bias. Any ideas?