I think this song means. She has a boyfriend who she loves. But she is having doubts about the relationship, and theres this other guy (maybe an ex boyfriend) who she isn't really in love with, but there's something there. And they kind of have a love affair so she can get it out of her system. And the doubts build up so much she feels she needs an emotional outlet. I'm probably wrong, but that's what I think it means! :)