Frottage Fantasies

I masturbate while having fantasies about frotteurism. I like the idea of a man brushing his cock against me, flaccid or erect. My ideal fantasy is being on a crowded train or bus and having a man stood behind me.

As the train/bus jerks about I feel something pushing against my ass, as this continues I turn around to see what it is and notice a man with a hard erect cock bulging through his trousers. I suddenly realize that it was his cock rubbing against me and I immediately get turned on.

He continues to push his cock between my ass cheeks as he knows I am enjoying it, I then begin to push backwards into his cock which pushes harder into my ass, all the while nobody is aware of this horny experience. He then begins to grip my ass as he pushes faster and harder.

I sometimes fantasize that he is a dirty old man rubbing his old hard cock on me and maybe I would help him out by stroking his cock with my hand. But the idea of letting this stranger touch me and press his cock against me with people around makes me cum so much!.. Any men ever do this?