The genesis of this question comes from the post
I don't know what Q&A he's referring to unfortunately.
asked Feb 28 '14 at 17:02
Regarding the Q&A about marriage, the main Q&A book that I'm aware of is Ayn Rand Answers. Happily, it has a very good index. I looked up "marriage" in the index and found several page references. I checked each of the references and found one that seems to match what the quoted formulation is referring to: the question that spans pp. 137-138, regarding Dagny's three relationships and the stability of romantic love. Ayn Rand's answer ends with the following (underline added):
Neither relationship [with Francisco and later with Rearden] was begun on the understanding that it was her final choice. And because she [Dagny] was not fully committed, when she met John Galt she was free to realize that he was exactly the type of man she had always hoped to find. She was emotionally and intellectually free to fall in love. If her relationship with Hank Rearden, say, had been different, and they were both fully committed to romance -- if they were married or living together permanently -- she would still have responded to Galt by finding him attractive and appreciating his value. But that appreciation would not have developed into real love. She would not have left Hank Rearden.
The general principle of "two people [who] have found the person with whom they wish to spend the rest of their lives" and being "certain that one's choice is final" also appears in Ayn Rand's Playboy interview in an excerpt that can be found in The Ayn Rand Lexicon in the topic of "Marriage."
The connection of this to the title form of the question isn't entirely clear to me.
If Rearden did not give up his metal, would Dagny have left him?
Apparently the question sees Dagny and Hank as romantic lovers before he gave up his metal, but giving up the metal was later seen by both Hank and Dagny as a mistake, so Dagny therefore became less romantically attracted to Hank. If this is the intended connection in the question, I see the context of the story as considerably more involved than that. The whole characterization of Hank, Dagny and Galt presents Hank as a very attractive and worthy lover for Dagny, but less ideal than Galt in many respects. Hank's metal is only part of that context, though certainly a major part. For Hank to have acted differently in regard to his metal would not by itself have changed the total context enough to make Dagny his lifelong romantic choice (or vice versa) in comparison to Galt, and it would have greatly undermined the impact of the story. Ayn Rand's aim was to present Galt as the fully ideal man, with Hank as a major hero who still needed to learn the nature of his destroyers. Dagny, too, needed to learn. Ayn Rand arranged the story's events so that Hank came very close to proposing to Dagny, and she might very well have accepted, but Ayn Rand spared Dagny from having to make that decision before meeting Galt, and also allowed Hank to learn of Dagny's changed state of mind after meeting Galt without Dagny having to tell Hank directly.
For Hank to have refused to surrender his metal would have had further ramifications on the story that would have added up to a major distraction from the central plot progression, such as having to inform Dagny of the blackmail before it became public, so that she could prepare herself for it; somehow revising other events in the story to make Hank's other actions and premises more consistent with the moral fortitude to refuse to surrender the metal; and so on. And then the contrast between Hank and Galt would have been far less dramatic.
In the comments, the questioner asks, in effect, if marital "commitment" is irrevocable. My understanding of Objectivism is that divorce is regarded as a completely valid and necessary option if a marriage turns bad.
The questioner asks specifically about Ayn Rand's own decision to pursue a non-marital romantic relationship with N. Branden while both of them were still married to others, without ending their respective marriages first. The context of Ayn Rand's life and circumstances are highly unique in this regard, and I consider Ayn Rand's actions entirely valid by the Objectivist standards she originated and advocated, within the context of her life (and the life of N. Branden at the time, as Ayn Rand understood it). It must be remembered that all knowledge is contextual, and that Ayn Rand regarded romantic love as an "exception-making value." (For references on "exception-making," see The Ayn Rand Letter, Vol. III No. 2; Journals of Ayn Rand, Part 3 Chap. 8; and some mentions in The Fountainhead.)
As far as I know, the first confirmed public acknowledgement of Ayn Rand's romantic relationship with N. Branden came in the form of a scene with Leonard Peikoff in the movie (and 1998 book), Ayn Rand: A Sense of Life, pp. 142-143 in the book. A vastly more extensive description of the whole context, financial and intellectual as well as romantic, is presented in the 2005 book, The Passion of Ayn Rand's Critics, by James S. Valliant. For Ayn Rand's role and state of mind, refer especially to Part One, Chapter IV, pp. 115-169, along with the additional information from Ayn Rand's personal journals in Part Two. One particularly significant fact about the relationship is that it was undertaken with the knowledge and consent of the two spouses. Valliant's book discusses whether or not the "consent" may have been psychologically coerced in any way, as well as where the relationship eventually led and why, and how it ended.
The Lexicon topic of "Marriage" explains Ayn Rand's view of marriage further, as follows (excerpted from the 1964 Playboy interview):
I think the question of an affair or a marriage depends on the knowledge and the position of the two persons involved and should be left up to them. Either is moral, provided only that both parties take the relationship seriously and that it is based on values.
Marriage is also a form of contract intended to be exclusive, unless both parties consent to one or both of them having a separate concurrent affair. According to Valliant's book, Ayn Rand apparently regarded having two or more romantic relationships concurrently as a near Herculean challenge, but psychologically possible if one can manage it. Refer also to the Lexicon topic of "Sex" for additional insights on romantic love.