Do you also fantasize about and/or act out situations in your head where you're being physically/emotionally abused in a relationship?

NO, it got nothing to do with a relationship and is quite damaging when it occurs, for a long time after, at times life. Rough sexual encounters will be differently, but only with someone I trusted never to take advantage of such situations beyond where I have my integrity with them afterwards too.

Depends what it is

Not something I think about, but I imagine it's more common that one might think.

No, I have been there in reality and can't imagine entertaining a fantasy about it... to me, it's a nightmare.