I am not a masochist, I just find the idea of you being a sadist, hot.
Why?
Ooooh! Oooooh! Oooooh! I know the answer to this one! Becaue intelligent, capricious women are mysterious and fascinating. Being a sultry dominatrix just completes the fantasy in an almost perfect manner!!
I still don't totally get why it would turn jman on, though, unless he wanted to somehow be a part of that fantasy. This isn't exactly like a visual; it's a concept. There's more thought behind it.
Then again, I guess assertiveness is something it's especially easy to expect folks to project sexual fantasies into... I've gotten the "you're probably into S&M, and you're prolly the dom" thing before. Also had it assumed I'd be into bondage. Also had it assumed I'd want to fuck guys up the ass while wearing a strapon. Thinking about it now, no wonder I've attracted so many fucked-up guys, and no wonder the most sexually reserved of my exes was so fucking terrified of me.