So This Is Ever After by F.T. Lukens

Carry On meets Arthurian legend in this funny, subversive young adult fantasy about what happens after the chosen one wins the kingdom and has to get married to keep it…and to stay alive.

Arek hadn’t thought much about what would happen after he completed the prophecy that said he was destined to save the Kingdom of Ere from its evil ruler. So now that he’s finally managed to (somewhat clumsily) behead the evil king (turns out magical swords yanked from bogs don’t come pre-sharpened), he and his rag-tag group of quest companions are at a bit of a loss for what to do next.
As a temporary safeguard, Arek’s best friend and mage, Matt, convinces him to assume the throne until the true heir can be rescued from her tower. Except that she’s dead. Now Arek is stuck as king, a role that comes with a magical catch: choose a spouse by your eighteenth birthday, or wither away into nothing.
With his eighteenth birthday only three months away, and only Matt in on the secret, Arek embarks on a desperate bid to find a spouse to save his life—starting with his quest companions. But his attempts at wooing his friends go painfully and hilariously wrong…until he discovers that love might have been in front of him all along