Yes. It's certainly true. It is very rare to read these kind of stories written by men - we are used only reading them from women. I think there is also an inherent bias that makes it more acceptable for women to write about things like this i.e. if a woman writes it is erotic and if a man writes it is considered "dirty" or unsavoury. I think this is a reflection of power structure in our mostly male-dominated societies. A woman writing about this is exhibiting sexual empowerment whilst a man is just doing what men do.
Also I think a man caught masturbating on a public bus would potentially get into a lot more trouble than a woman. For one thing it would be much harder to hide it (obviously) and the criminal connotations would due to the situation be more serious.