I'm not telling my kids to never hit girls

My son just started kindergarten and before sending him off I sat him down and told him to never hit a girl. He asked "what if she hits me". My son is huge for his age and is quite the lovable gentle giant. So I told him to walk away and go tell a teacher. My wife thought that was the right answer but I don't think it is. That's basically saying hit a girl and nothing will happen to you. I changed my answer to never hit a girl UNLESS it's in self defense. But if we find out you hit a girl and it wasn't in self defense there will be consequences. I feel like that's a better answer.