I think of step-throughs (including mixte frames) as unisex, and I ride my wife's mixte all the time.

The U-frame seems to be more common than mixtes in newer bicycles like the Electra Townie, Raleigh Venture, and the Breezer U-frame line. I'm a fan of the vintage Raleigh 3-speed mixtes, but a mixte with more gears might be desirable as well.