But, I bet you that in his mind, saying "girl-friend" is a way to have it both ways - if you challenge him on it, he can say "no, I hyphenated, see, I'm acknowledging that you are right and they weren't my sweethearts", but later he can say that you "accepted" his version of events, because you allowed him to keep saying "girl-friend" which is homophonic for "girlfriend".
