It is because females have the chance of becoming pregnant and therefore less able to defend themselves and infants (in addition to having muscles that are less strong than males) human society requires chivalry to remain healthy. It is expected in human tribes that males are protective toward females and this is especially strong in romantic relationships. Without protection females are guarded against relationships and pregnancy. In this dog-eat-dog society where all are atomised and expected to have the ability "stand alone" society disintegrates, family life falls apart. Chivalry is not a nice "add on," but vital.