The only real disadvantage I can see with "women's" bikes is that there isn't as much room to attach things to the bike. I'm female and I bought a "men's" bike only because there was easily enough room for a water bottle cage, pump and lock bracket. Attaching all those things would have been nearly impossible on the step through bikes I looked at.
That makes sense. I don't know how most step through frames are, but I do enjoy being able to mount a couple of water bottles to my diamond frame bike.