In ethics, it is often pointed out that just because something is the case does not mean it ought to be. So, just because women are paid less than men, does not imply that they ought to be. Kant argued that morality was wholly based on Reason alone and that we could tell if something was morally acceptable or not by asking ourselves if what we were considering doing should be a univeral law; something everyone did all the time.
So, according to Kant, before you tell a lie you should ask yourself "Would I want it to be a universal law that everyone lied?". Reason tells you that the answer is NO because if everyone did, then one could not even state the universal law (everyone should lie) without breaking it. This law leads to absurdity and so is immoral.
Kant argued that you could tell if something that is the case is morally correct by applying this universal law or test to it. Thus, it is the case that women are paid less than men overal. The question for Kant would be is it moral? Let's see. Would that I want women to be paid less than men? What are the ramifications of this question? Is it reasonable? Does it lead to a contradiction? If so, then Kant would say it is immoral.