I don't know what the first thing a girl COULD do to you is, (Other than torture you in the most vile manner with any tool she could get her hands on.)
But the worst thing that happened to me is when a ex-girlfriend punched me so hard in the nose that I started bleeding, and then broke up with me.
We later went out again, which was bizarre.
Why had she done that? Because I kept messing with other girls to tease her.
After that experience I learned to stop.
