From my personal experiences, I see this song as being with he is with a girl that is cheating on a current boyfriend with him. He wants her to leave her boyfriend for him, but only if she loves him and not just for the hell of it (hence the 'If you're not in love stay where you are'). Also, it could be that everyone around them except her boyfriend knows simply from the way they act together (We were such terrible liars, we were such passionate lovers). I can easily see it being the other way around too, this is just the way that I see it relating to me.
From my personal experiences, I see this song as being with he is with a girl that is cheating on a current boyfriend with him. He wants her to leave her boyfriend for him, but only if she loves him and not just for the hell of it (hence the 'If you're not in love stay where you are'). Also, it could be that everyone around them except her boyfriend knows simply from the way they act together (We were such terrible liars, we were such passionate lovers). I can easily see it being the other way around too, this is just the way that I see it relating to me.