I'm pretty sure this fic was omega Charles and alpha Erik. I didn't read it too closely, but I think Charles was being married off in a political arrangement in some kind of religious ceremony, and Erik saved him from it at the last minute; Erik may have been the one marrying him. Charles is confused and keeps trying to be a perfect omega for Erik, who is frustrated and confused that Charles doesn't seem to understand...
If anyone could help me find this, I'd really appreciate it!