Are we allowed to tell women they can’t wear pants at work?

Q. I am an HR director for a traditional, conservative company and have run into a new issue. When onboarding employees, I always explain our preferred dress code: we prefer women to wear skirts and dresses with pantyhose to work because many of our senior partners are old-fashioned. A new female employee said she would prefer to wear pants. Can we require her to wear a skirt instead?