In today's world, there are plenty of experienced and qualified women for the boards of corporations. Therefore, quotas for women aren't necessary on corporate boards, and such an idea is misguided. More and more women will continue to break into the corporate world and find themselves on these boards in due time.
If such a rule would be in place then the amount of selective hiring that would occur could be detrimental to a companies quality. Although fairness in the workplace should always be top priority it is not fair to try and force this. The company should be able to decide what they believe is best for the board and if a male candidate were to be more qualified but not hired because the board needs to fill a quota than that in itself could be a new type of discrimination and defeating the purpose of the rule in the first place.