"doesn't it make sense that women make less money when, on average, it costs more to hire them?" Why is there an assumption that a women should cost more to hi

"doesn't it make sense that women make less money when, on average, it costs more to hire them?" Why is there an assumption that a women should cost more to hire? Don't men also take paternity leave? Why is it assumed that a woman must be solely responsible for childcare? No one seems to worry that a man might have children when hiring.