Why is it considered by others to be a bad thing, to see women as lesser and to keep them down, just because they're women?

Let me give you an easy out here, and tell you my assumptions. I am assuming it also only took you a minute to look up the definition. And after reading it, then couldn't come up for a reason for others to judge that kind of behavior.

I am kind of stumped of how to actually reply to this, to me it is just pretty obvious that any kind of prejudice or bias like that is strongly unwanted.