Trick Force or Choice
Danny and Phantom are different people; they are just fused together after the portal opens up on top of them. As Phantom was crossing the ghost zone, the portal opened up on him as Danny turned it on, fusing them together. They are still their own people just sharing the same body. Danny has control of the human form, and Phantom controls the ghost form.
Spme background, Danny died when the portal opened, and the only reason he's still alive is because of Phantom's ectoplasm. They figure out how to keep Danny alive without him, but Phantom is afraid of losing him.
Takes place after defeating Pariah Dark.
Phantom becomes the new king after defeating Pariah Dark. These are three ways Phantom gets to keep Danny either as a friend or something more. Either by trick, force, or by choice.
Ideas:
Trick: Whoever is given the royal consort pendant by the king is the official consort. Phantom doesn't want to lose Danny and thinks the only way to keep him around is as a wife, so he doesn't tell Danny the meaning when he gives it to him. Danny doesn't learn about it until a DC crossover. Got inspired by dcxdpdabbles The Royal Consort.
When confronted, Phantom pretends he didn't know. They decide to keep up the charade because their marriage is seen as a treaty between the two realms. Danny later finds out the truth, and there is a lot of angst involved.
Force: Phantom protected Amity Park for years and wants something in return; Danny. Phantom uses the ghosts he controls to grab Danny and puts the pendant on him. Like Sam's crown, only the king or someone of equal power can take it off. Danny can either be stuck by his side or gain his own powers to escape. Inspired by Anastasia's villain song dinner scene. The song has nothing to do with this. I was just listening to it when reading The Royal Consort, and the idea was born.
By choice: After Phantom is crowned king, he gets many marriage proposals and rejects all of them. The officials are stubborn, and they refuse to listen to him unless he gets married. They think they have backed him into a corner, but then Danny finds out. He decided that since Phantom protected him and his town for years, it's his turn to protect him from the greedy officials and became his consort. When they try to pull the "two men can't have a baby" argument, then they bring up Dani and name her their heir