Why is "holding the door" even argued?

[WhaWha]

If you're a man and you see a woman right behind you, hold the door for her.
If you're a man and you see a man right behind you, hold the door for him.
If you're a woman and you see a man behind you, hold the door for him.
If you're a woman and you see a woman right behind you, hold the door for her.

It's called common courtesy, I don't see why gender even has anything to do with it. I hold the door for everyone, woman, man, child, etc.
The Man: You are way overgeneralizing, I am friends with lots of women, and I don't know any that fit into any of those categories you mention.

 

[PunIntended]

It's because everyone seem to think that chivalry is dead. I'm a girl and I always open the door for people. I would say 'Ladies first,' even though they're not a lady. People may think it's weird, but I'm just being nice.