I interpreted it this way:
Superman is a metaphor for a "too perfect man", the kind of man that people like and respect and adore. Certainly this "superman" is liked and adored by his "ex". By making this man a "perfect man", the singer has made him less relatable, because in the presence of perfection everyone would feel inferior and inadequate (which is how the singer feels from being selected against in favor of this other man).
The "say your stupid line" part is a little more vague but I understood it mean that the singer anticipates a confrontation from the "perfect man". This man is a "perfect man" who would fill a kind of "tender defender" role to his ex. As a tender defender, he would care about his partners feelings and the uncomfortable nature of the singers continued adoration towards the girl. The singer is not moving on and so the tender defender would come along and say the kind of thing a tender defender would say, a kind of "line" that fits the role. e.g. "hi singer, I think it is time to move on friend, she doesn't appreciate this" etc. This kind of line would further insult the singer, who actually wishes that he was the one saying that line to someone else.
What does all of this mean? well, it is much easier to dislike a too perfect "superman" character who mindlessly says his perfect "lines" than it is to hate a fellow human who is simply pursuing a fulfilling and meaningful relationship with someone. Presenting the lover guy this way allows the singer to maintain hope that his ex girlfriend might one day come to hate the new guy as he does, and thus, their relationship may be still recoverable. That is what this song is about, not giving up on a girl, and hating the guy she is now with.
I could be wrong, but that's how I understood it :)