I didn't know she was in a relationship until several months into ours. It was like that episode of the office where Michael Scott realizes that he's the side dude and in astonishment says, "I'm the mistress."

It was during a study abroad year so her boyfriend was "back home" and thus she never saw fit to mention him. She just kind of had to when, after a couple of months together, he flew into town to surprise her for a weekend visit. I assume she was surprised. I was very surprised when I walked up to her at the bar, put my arm around "my girlfriend" and then she introduced me to her boyfriend. My jaw dropped and I was like, "Oh hey. Nice to meet you." Then I left as quickly as I could.