He invited me over to 'his' apartment, showed me his divorce paperwork, and ultimately things got sexual for a couple weeks. I (stupidly) believed him, we exchanged numbers, started talking daily. 'He gave me this sob story about how Kelly wanted a divorce and was taking his kid from him. Almost a year after we broke up, I was back in town on summer break from college and ran into him at my summer job. When we were off he would date this girl Kelly, and when we broke up for the final time he went and married her. The guy was an old boyfriend from high school, we dated off and on.
It's possible to unintentionally be the other woman.