a "men's" bicycle typically has the top tube of the frame parallel to the ground, while a "women's" bicycle usually had a top tube that is slanted lower at the seat end, so a lady doesn't hit herself in the balls if she slides forward off the seat
I rode a touring bike with a 'mens' frame for a while. (I always wear trousers) One day, an old chap saw me unlocking it outside a shop and asked if it was mine. When I said yes he asked "but how do you... Get on it?" I swung my leg over the saddle, said "like this" and rode off. I think he genuinely thought women weren't capable of doing so.