It always surprises me that women don't understand the social stigma of a man exerting maximum force when physically competing with a woman. You have to understand men are brought up to respect women and treat them with extra care. It is seen as socially unacceptable between male peers to "go all out" when competing with a woman in a sport. I believe this social stigma is the underlying cause for taking it easy on a woman.
Tennis is a little unique in that below the pro level, the amount of skill/technicality a person has determines who is better. The more physically imposing player does not necessarily have an advantage. Despite this fact, I and most guys will still take it easy on women because it is pretty much hardwired in our brains. Now if I'm playing a D1 girl or higher... no mercy.