Difficult and troublesome to enforce - what happens if one person (regardless of sex) is a hard worker and so has been given pay rises over the, say, three years they've been at the company but another person does the minimum and so has only got one pay rise in the last six years? Should they be paid the same?
Ultimately, as I understand it, disparity in pay between men and women is not simply because women are being discriminated against in the workforce, although I would argue that discrimination does still exist. Instead, it's largely because women take time off to have children and then usually go back to work part-time after the birth of the child. As a consequence, they tend to miss out on promotions and other opportunities, whereas men who have a largely uninterrupted work history don't miss out on those opportunities.
If we want to get rid of pay disparity, we need to look at changing societal attitudes - make it more acceptable for men to have time off after the birth of a child, give men access to parental leave and so on. Until that happens, pay disparity will still exist.