I don't get why people seem to fear that Oda uses the classic "betrayal over a woman/with a woman involved" storyline.
It's one of the oldest and most classic storylines in all of drama.
It was used in ancient Greece (Ilias - story about Paris and Helena), by Shakespeare (Othello) and even in modern manga classics (Berserk - Guts and Griffith over Casca).
Given the fact that Oda loves to take ideas from all those kind of sources and is a bit of a literature-nerd himself, I totally see him go this route:
Joyboy and Imu were once close friends, two of the mightiest inhabitants of the ancient kingdom. Imu secretely loves Lily and wants to propose, but to his shock she falls in love with Joyboy, who is kind of a dumb Luffy-clone and doesn't realize how this hurts his best friend. Imu develops pure hatred towards Joyboy and plots to tear down everything Joyboy loved, starting the void century. Lily manages to escape and hides on a remote island in the Calm Belt, later known as Amazon Lily. The other Nefeltaris have heard the rumors about Lily/Imu and refuse to become celestial dragons under him.