So what am I missing? Is there a certain time that you have to speak to him? I did the 3 quests in the temple of Mara, then decided to speak to Maramal. At no time have I seen an option to ask about marriage. Anyone have an idea what I'm missing?

Do you already own an amulet of Mara? (Not one bought from the temple, one of the ones you can find around the world/on enemies) If so, you'll need to get rid of it first before being able to get another one. (Also, I believe one of the quests there give you an amulet as well, if you don't want to spend the gold buying it.) Hope this helped!

The marriage topic is part of the dialogue chain where you ask about the temple of Mara. You don't have to purchase the amulet of Mara from him; I just had the one I found on day one of the game and it worked fine.