If he dies in the final battle, you aren't queen yet, so you can't reign.  You aren't married to him.  If you both survive, the text at the end says that you marry him six months later.  Anora stays queen if he dies.  

The Dragon Age wiki says that you can make a political marraige even if you aren't in a romantic relationship with Alistair.

http://dragonage.wikia.com/wiki/Possible_Landsmeet_Outcomes