Her higher value, naturally.

Think of the thousands of generations when people lived in extended families or small tribes. If all but a small number of the males were wiped out the clan could survive and make a rapid comeback given sufficient numbers of fertile females. The situation would be quite different if all but a small number of the females were wiped out, no matter how many males there were. The fundamental difference is that, in nature, sperm is plentiful while eggs and wombs are not, so the clans that tended to survive were the ones whose instincts protected the female at the expense of the male. As this natural force operated and its effects accumulated over long periods of time the successful human genome became progressively more feminacentric: female wellbeing took, and takes, instinctive precedence over male wellbeing. One might say that those whose instincts promoted male wellbeing as equivalent to or above female wellbeing are nobody's ancestors.

The net result is a bias in the psychology of modern humans towards instincts that look after the well-being of women more than men. This is so built into our tribal brains that to be a non-conformist and to not operate in accordance with its dictates will quickly get you outcast and hated. In fact people's built-in detectors are calibrated very sensitively on this sort of issue, always on the look-out for defectors even though many no longer live in a clan-based society.
Moreover, people feel a strong compulsion to intervene and enforce the status quo. At a subconscious level they become disturbed and unhappy if they perceive this instinct being ignored. After all the psychological bias toward feminacentrism would mean little if there were no built-in automatic corrective mechanisms to go with the deviance detection mechanism. The clans that survived better were the ones that enforced feminacentrism by making sure that any individuals that didn't feel it strongly were cast out. There are many similar instincts in the human psyche.
[Thanks for much of the above goes to http://www.martianbachelor.com/Science/, a highly recommended site.]

Those who promote first world liberal principles such as gender equality are seldom aware that they and their community labour under the influence of a deep instinct that fatally undermines this moral framework. You'll never be equal with your neighbour if, instinctively and unconsciously, both you and she value her more. My 'Prove it!' post (below) demonstrates just how strong and unconscious this instinct still is in western civilisation.