If that person does worse at their job for any reason then I wouldn't pay them as much as a person doing it better than them.
But I wouldn't immediately assume that all women will be worse than men so by default I will pay them less, because maaaaybe they aren't as good as men at doing it.
And when we're talking about jobs in businesses and offices and such, there's even less reason to assume a woman is less equipped for the job than a man.
They won't be doing heavy lifting in an office. I can't just assume that the generally physically weaker woman will be worse than the generally physically stronger man and typing on a computer and filing paperwork.
You're theory is well intended, but easily abusable. The current american legal system works in a flawed way, so that the loser of a court case does not have to pay the legal expenses of the winner. keep this in mind. Say I am an employer, and you are a female looking for a job. If I employed you and paid you less based on work, you might sue under this equality law, saying that I was being discriminative of your gender. This may not be fact, but you have sued me. You will probably lose, but under this system I still owe my lawyer for defending a pointless case. Now, I am drowned in legal expenses and might have to close down. That is the fear of employers under this law. They call people like this "Lawsuit Bombs" because of this. Not every single female is like this, no sir, but the ones who are like this scare everyone else out of hiring all of them.
And now
more females would be out of work.
We must all agree that this senator is absolutely handicapped though.