Why is my male turtle nippling on the female?

For three months now George (the male turtle) has been biting or threat biting Martha (the female). At first I thought this was a normal turtle thing. But the biting has lead to a "huge" cut hidden by her leg. So, I've removed them and Martha's leg is healing nicely. I was hoping someone could tell me why George is behaving this way and if removing them was a good idea?