Because in Victorian times when most of these laws were at their height women were not considered to have any sexuality at all. They certainly weren't suposed to enjoy sex. They were to lie back and think of England.
Plus to a hetrosexual man two women together is not automatically a turn off. Whereas, the thought of another man, doing that, touching them there, well, that's just icky. They know that's wrong. And men write the laws.