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Common courtesy is not the same as chivalry, as common courtesy is expected by both genders. Chivalry is only expected by men. That means chivalry is a gender role. Since chivalry is a male gender role, it is therefore sexist according to modern feminism which claims to fights against gender roles. Most feminists would be very offended if they were told they needed to run head opening all of the doors for men, pulling out the man's chair so he can sit down, etc just because they were born as girls.
Any role placed upon the woman is seen as degrading to women, while any role placed on the man is viewed differently. Some even flip it around so much they claim the male gender role of chivalry is actually an insult that means the woman is too weak to do those things herself.
Feminist ideals were created by people with a lot of gender bias. You put the same restraints on both genders and they will come up with different answers. This is why they think women doing something for men because of her gender is degrading, but when a man does something because of his gender for a woman it is never seen as degrading for the man. They are not holding us to the same standards. Everything is about twisting reality to make women look like the victim instead of looking at it fairly and logically.