From TE's Guide to Women's Bikes:

A "mixte" frame and a "women's" bike frame are not at all the same thing. A true Mixte bike frame has two parallel bars that run from the handlebar stem, pass on either side of the seat post and run back to the rear hub. A mixte frame is as strong or stronger than a man's frame, though it weighs a bit more.
http://www.teamestrogen.com/wsdBikes.asp