So I had a conspiracy theory.
It goes liek dis :
The only reason the handsomest character in all Skyrim is not marriageable when every fail, weak, and puny is available is because Bethesda hit upon it as a marketing tactic.
If they allowed us to marry him, then nobody will make all the Hadvar fanart and take 320948712093810 Hadvar screenshots instead of actually playing the game. We will moon around Riverwood so much that the game time will be extended. To top it off, all those screenshots of Hadvar?
Yeah, great publicity.
I'm just sayin'