Here's why:
* Helios' Sacrifice: Helios's ultimate fate is to sacrifice himself to save Sailor Moon and the Earth from the forces of evil. He merges with the Golden Crystal, effectively becoming its guardian.
* Rini's Future: Rini, who is Sailor Chibi Moon, is still a young girl at the end of the series. While she and Helios have a connection, they are not romantically involved.
While some fans might imagine a future where Helios returns or Rini grows up to marry another, the official canon of *Sailor Moon* doesn't depict such an event.