A traditional "lady's" bicycle (or a mixte) sacrifices the strength and rigidity of a horizontal top tube in order to make it easier for the rider to wear a skirt.

The sexual politics of this is clear - inconvenient clothing is presumed to be more important to women than having a bicycle with superior performance. This is just another example of the idea of suffering for style, or form over function, unless of course the practical advantages of wearing a skirt outweigh the practical advantages of having a stiffer, lighter bike.

As for the horizontal top tube on a "men's" bicycle, this presents no danger to the testicles as long as the bicycle is the correct size for the rider. You should always be able to straddle the top tube with both feet flat on the ground - if you can't, you need a smaller bike.