I've always thought that when Ennis asked this question he was thinking about Jack and the love they shared. I have no doubt that he had realized how much he loved Jack and how much Jack loved him, and he finally realized that nothing is more important than that.
But it could also be, like kindred posted before me, that he was thinking about his own marriage. He married a woman he didn't love, not really, and that's why his marriage failed. that's why he couldn't marry any other woman, and I suspect he had serious doubts about the whole marriage institution - "once burned"... Ennis doesn't want his daughter to be in this kind of relationship. He doesn't want her to have the kind of marriage he and Alma had. He wants her to marry someone who REALLY loves her, and have the kind of happiness that he never had.