It is Spander.
It is kind of like a future AU fic
Spike is like a leader of this small ship in space, and he needs to choose a mate, and he decides on Xander. But when Xander finds out, he is hell bent on getting out of it. He couldn't get out of it, and ends up being in a forced marriage with Spike and he tries to make Spike regret it. He wears a wristband, or a collar of some sort so Spike always knows where he is or something, but towards the end, Spike gives up, and takes it off Xander while he is asleep, and the next day they go to earth so Xander can see his friends, but Spike leaves Xander there, cause he wants him to be happy and free. Xander freaks out, and starts crying and calling for Spike in their mind link, but Spike trys to shut him out, and it starts hurting Xander cause he is trying so hard.
I really really want to find this story, so please help me.
EDIT: It's NOT the story called 'Breaking Free'
Spike gets to choose his mate, by looking at videos of the people he could choose from. He also had the choice of Angel too, but he wanted Xander.
He is the captain of the ship.
EDIT FINAL: It is called A Whopper of a Tale by Virtual Personal and Nashmaveric in the series 'Time After Time'
Thank you for helping eyezrthewindows.