The subject of the song is dead and that's why the song is about space. The narrator cannot get to the memory of their lost love and instead must watch from a far. The world has moved on and have forgotten them but not the narrator. Burying their love in the moondust means exactly that, the narrator must create an ugly scar on the face of the planet to conceal the truth, the song is about their tormoil about moving on. By placing themselves on the moon the narrator is far away from the rest of man kind. They're isolated and they intend to stay that way. Mankind cannot get to them "nothing can breathe in this space" "Building this house on the moon" also nothing can live on the moon this furthers the idea of isolation. It is the narrators chosen prison.
Great insight.