This is a song about a guy Rufus met in Greece. The story goes that Rufus met him and they talked, but when Rufus tried to make out with him, the Greek got freaked out. "Save your poison for a lover who is on your side" is an example of Rufus telling him, if you're going to talk about loving men, act like it or save your poison for women.
This song is about Wainwright's fear of success. He fears what his future self will be and is worried he will fall into the showbiz stuff and end up wandering around New York drunk, a nobody.
