According to Osho, 'the honeymoon is over' means the fantasy-projection of love has collapsed; the dream ends and real, down-to-earth relating - marriage - begins. The higher the romantic high, the sharper the disillusionment. Fantasy thrives at a distance; meeting exposes reality. Socially arranged bonds "succeed" by skipping the dream, settling into practical coexistence rather than extreme love or hate.
It means the pretend, dreamy part of love ends, and the real everyday work of being together begins.