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"You're crazy!" he exploded. "I can't speak about what happened five years ago, because I didn't know Daisy then—and I'll be damned if I see how you got within a mile of her unless you brought the groceries to the back door. But all the rest of that's a God damned lie. Daisy loved me when she married me and she loves me now." (7.246)
Notice how Daisy's love is like a possession to Tom? He sees marriage as a system of ownership, and he's all about controlling access. Gatsby can bring groceries to the back door and drive her around, but he can't have anything more.
He nodded sagely. "And what's more, I love Daisy too. Once in a while I go off on a spree and make a fool of myself, but I always come back, and in my heart I love her all the time."
"You're revolting," said Daisy. She turned to me, and her voice, dropping an octave lower, filled the room with thrilling scorn: "Do you know why we left Chicago? I'm surprised that they didn't treat you to the story of that little spree." (7.251-252)
Tom expects a lot more from Daisy than he does from himself, but this was just par for the course in upper class marriages of the time (and a lot of time previously, too). Men got to play around; women got to produce heirs. It's a bargain.
"Oh, you want too much!" she cried to Gatsby. "I love you now – isn't that enough? I can't help what's past." She began to sob helplessly. "I did love him once – but I loved you too." Gatsby's eyes opened and closed.
"You loved me TOO?" he repeated. (7.264-266)
Gatsby seems more upset by this confession than by the fact that Daisy's actually married to someone else. Marriage doesn't mean much of anything; it's just a dying social system. The feeling is what matters.