Can you marry Elliot in Fable 3?

Yes. You have to play through as a female. At the very beginning of the game, when your brother forces you to choose between Elliot's life or the villagers, you have to choose to spare Elliot. Later, do the "Kidnapped" Quest in Bowerstone Industrial and you find out the kidnapped fiance is Elliot. At the end of the quest, you can convince him to dump his fiance and marry you instead.