So the inclination to protect our women from bad men provides a genetic advantage, whereas protecting our men from bad women would be genetic folly. By selecting those sensitivities which promote our genes, nature programs us to be outraged by men exploiting women but not by women exploiting men. The traditional double standard which allows casual sex for males but not for females applies only to those within your clan. For outsiders, it reverses itself. That is, the double standard allows a girl from elsewhere to have casual sex with a son, but stands adamantly against a boy from elsewhere who wants casual sex with a daughter.
Chivalrous sympathies can provide additional genetic benefits.
A man who stands up for a mistreated woman against her husband may gain favor with her or with her women friends, any of whom can transport his chivalrous genes into the next generation. A man who stands up for a mistreated man against his wife can lose favor among her women friends, reducing his chances of mating and imperiling his genetic future. Men are programmed to avoid such huge genetic risks. Women who stand together also set the standards of conduct for their communities, thereby protecting themselves. Today, with modem birth control, many accept it when a woman friend sleeps with her lover or even lives with him, so long as she is content with the arrangement.