Swap Girlfriends

Your girlfriend Anna has a kind of weird pool of friends. They always told you stories you didn't know if they were jokes or real. But the yesterday there was the annual swap party. You didn't know what it meant before you got there. There were three other couples: Jason and Mary, Steve and Daniela, Kurt and Sarah, you and Anna. Every guy had to pull a photo out of a box and you pulled Sarah's. You asked what that means and Anna told you that you were coupled with Sarah for the next 12 months. You didn't believe her, but finally when everyone left, Anna left with Steve and only Sarah stayed.