Sex & the Single Woman: I’m the Other Woman?

Recently, one of my roommates hooked up with a friend of a friend. She knew him well enough to know that he was a decent guy, but that’s about it. Nothing came out of it relationship-wise, but that’s okay because she wasn’t really looking for anything. Apparently, neither was he.

After the hookup, however, she came to find that has a girlfriend. Whoa–where did that come from?

What is a girl to do when she finds out she is the other woman? She had no idea going into things that he was a taken man, yet now she has guilt upon her shoulders.

When meeting guys should we just be trusting them to tell us that they are off the market? What happens when we accidentally become the “other woman”?