He wrote Helen that a young writer needs desperately to live with someone and he had decided that he wanted to live with her; even marry her, he offered. (6.55)
Garp's early approach to marriage is immature, to say the least. He should just thank his lucky stars that he ends up with a woman as cool as Helen.
In the case of Garp and Helen, they hardly knew each other but they had their hunches—and in their stubborn, deliberate ways they fell in love with each other sometime after they had married. (7.12)
Listen kids, we wouldn't recommend marrying anyone before falling in love with them first, but it seems to work out okay here.
He would always call her "the wisest of my life's decisions." He made some unwise decisions, he would admit; but in the first five years of his marriage to Helen, he was unfaithful to her only once—and it was brief. (7.117)
Although Garp has no shortage of praise for Helen, you can already see the cracks in the surface. Do you really think that it's an achievement to wait five years before starting your first serious affair?
Sometimes it panicked Garp that Helen seemed to want him to stay at home and "just write"—because that made the domestic situation the most comfortable for her. But it was comfortable for him, too; it was what he thought he wanted. (9.57)
Garp worries that his unconventional marriage won't last because it doesn't match the typical male-female dynamic. But Garp and Helen aren't typical people, so it's doubtful that a typical relationship would work for them.
"You're too 'sorry' all the time," Mrs. Ralph said. "What a sorry man you are. Except to your wife," Mrs. Ralph said. "You never once said sorry to her." (11.150)
This nugget of wisdom comes from the woman that Garp considered sleeping with just moments ago. Ouch. But it's true: He's lost sight of Helen and doesn't realize that he's suffocating his marriage.
When Duncan finally went to his room, Helen and Garp were left with less than half an hour before Walt would wake up. But Helen had the names of her enemies ready. There is plenty of time to do damage when you know where the war wounds are. (11.254)
Of course, Garp and Helen love each other. But love breeds familiarity, and familiarity is much-needed before launching a major offensive.
And for how long had she really been irritated by Garp's routines and habit? She didn't know. She only knew that she noticed she was irritated by them almost from the moment she read Michael Milton's questionnaire. (12.91)
Garp isn't the only one to grow restless in the marriage. Michael gives Helen attention and Garp doesn't—it's as simple as that.
Alice and Harrison Fletcher would remain married, through thick and through thin—in part, their marriage lasted because of Alice's difficulty with finishing anything. (19.59)
While Garp and Helen love each other but screw up at times, Alice and Harrison are too passive to end a marriage that was never meant to work.
[Helen] would have lovers but never remarry. Each lover suffered the presence of Garp—not only in Helen's relentless memory, but in the articles of fact that Helen surrounded herself with. (19.69)
Although Garp wasn't always the best husband, he was always a lot of husband. It's hard for another man to match the sheer size of his presence.
He came home [...] and found a very lonely transsexual living in his New York studio-apartment. She had made the place look as if a real artist already lived there. (19.273)
So Duncan ends up getting married for the same reason as his father. Just as Helen makes Garp feel like a real writer, Duncan's wife makes him feel like a real artist. Love, sweet love.