Your last question is a good one, and the pithy answer is, because it's against the law. But the real answer is that it's against the law because gender discrim

Your last question is a good one, and the pithy answer is, because it's against the law. But the real answer is that it's against the law because gender discrimination and sexual harassment (and a single act can be both) are such a systemic problem that we had to create laws to address it specifically.