I've had this happen to me as the guy supposedly stealing the girlfriend.

Except that until her boyfriend came looking for a fight, I had no idea she was with someone else and she made no indication as such.
And this was after going on several dates with her.

Suffice to say, I haven't spoken to her since. I'm not a cheater and I don't want to facilitate cheating.

And when I notified him of the fact I had no clue they were together, he was level headed enough to not be pissed at me anymore for what happened.

Him, I have actually seen and spoken to since then.