If you are really convinced that women get less than 80% of what men get for exactly the same work, then let me suggest an easy plan to get rich: Start a business in a labor-intensive industry, hire only women, and pay them 10% less than you would pay a man. You are then paying them 10% more than they would get anywhere else, so you should easily be able to get the most qualified people available. But even at that your labor costs will be 10% less than companies that hire primarily men. You should easily be able to undercut the competition and still make a bigger profit than anyone else.
Why aren't the liberals doing this and making a bundle? Hmm, maybe there's a flaw in the initial assumption that women are underpaid.