I've been in just one exclusive dating relationship, in which we actually referred to each other as boyfriend and girlfriend. The others either never turned into anything serious, or weren't exclusive. However, I've still only really dated one guy at a time, so after the break-ups, I referred to them as boyfriends (or rather, ex-boyfriends). It's easier that way than saying long explanations like "the guy I was seeing at the time" or "the guy I dated off and on for 8 months."
Makes sense. Kind of like how there was officially no World War I until there was a World War II.