Long, long ago, in a time that you've only heard about, there was such a thing called "common courtesy". It was a magical thing that made people act nice to each other. People still believed in the difference between men and women and each gender had roles to play. Women weren't inferior to men exactly - but because they were the ones who worked outside the home to support their families, they were considered the head of the household. This is how women were perceived as being "inferior" - but such was never the case. Their individual roles were not to compete with one another but complement each other. Since the dawn of time, men have always been the providers and women have always taken care of the house and children. What the man wasn't suited to do, the woman was - and vice versa. Thus their capabilities complemented one another. Having doors opened for them was one of the perks that went along with the role. Centuries ago, men would even take off their jackets and place them over a puddle so women wouldn't have to get their feet wet. In today's society, all of this has changed and no one is any better for it. People are messed up in the head because their world has become one of "what's in it for me?"

It's just good manners.