@kenreed it's not head cannon. It's stated outright in the story.
we don't know if he had romantic feelings towards the knight
Chapter 3: "I've been waiting to be with Luka for ten years..." Now granted, it could have just been horniness. But if you have a close relationship with somene, and find them attractive, maybe you ought to explore that before marrying a stranger because it's "expected" of you. I mean, we don't actually know that he has romantic feelings for her now either, by that measure, but "cheating because you're horny" isn't very praiseworthy either.
Plus the princess was traveling with them so whose to say he knew more about the princess than the knight we don't know
The princess wasn't travelling with them. Again, it's said right in the story, Chapter 1: "If you save the princess who was taken hostage by the demon lord". The princess was captured, and the hero party rescued her, and on their return he married her. Clearly the return journey would have been much faster than the quest to save her, since they had to spend time levelling up and such and returning would have been essentially just warping back.
Maybe he wantd to be king but lost the desire.
Again, chapter 1. When the king tells him "You could take my place as the king of this country" his reaction is "...seriously? I... a king...". This is not the reaction of someone who wanted to be king and took steps to achieve it, but then grew disillusioned. That's the reaction of someone who never thought two steps ahead and has just realized that he is about to reach the obvious destination his path led to.
So no, it's not head canon. If anything, your comment about the princess travelling with them shows you haven't paid any attention at all to even the basic setup of the story. I know the female knight is distracting, but I'd hope at least the basic setup of the story would make an impression.