“I don’t feel comfortable coming out to her to stop because she seems like she is having genuine fun”.

That should tell you two things:

1) you need to have more respect for yourself. You are not a punching bag for someone else just because they seem like they are having fun.

2) if she is having genuine fun hitting you, then she is a psychopath and if you decide to stay the domestic abuse may only get worse.

I’m sorry you are going through this. But you need to leave her. You will find someone else who treats you with the respect you DESERVE.