(covers information from several alternate timelines)
At 24 years of age, Archer proposed marriage to Margaret on the night before he graduated from flight school in San Francisco, in front of her apartment on Westgate Avenue. To Archer's dismay, she declined, stating that she did not wish to become a "Starfleet widow."
In 2165, T'Pol related this story to Archer, as a way of proving that he was not the victim of an elaborate deception, when she told him that he had been infected with a series of interspatial parasites twelve years previously, which inhibited his ability to form new, long-term memories. As Archer had never told the story to anyone else, this effectively convinced him that T'Pol was telling the truth. The timeline in which this occurred was negated when the parasites were eliminated, neutralizing their presence in the past as well, changing history in such a way as the infection never occurred. (ENT: "Twilight")