My girlfriend slapped me once.

And you know what? I put up with it. I didn't hit her back. I wasn't passive-aggressive about it later, either. All I did was make a slightly shocked "Y U do dis" face.

She said she felt like a dick for doing it later -- she said that with the reaction I had, slapping me felt like slapping a puppy.

Unnecessary violence is bad, period.

I think anyone hitting anyone is bad.