How did I miss this?

I wish I could see the actual thread so I could see the point that person was trying to make, but just taking what you have here, I am pretty inclined to disagree with that point and generally agree with the argument you are making. Obviously this is playing about on the edge of a hypothetical, but I think it's a really interesting question, and one that has a lot of larger importance.
If we go back before the beginnings of the AIDS crisis (1981) there's really only 15ish years of gay identity/activism. The Stonewall uprising/riots in 1969 are considered the beginnings of the LGBTQ Movement. Before that you have some stirrings of similar confrontational activism like the Compton cafeteria riots as well as 'quieter' forms of social activism like the Mattachine Society and the Daughters of Bilitis. Stonewall does kick off a significant change, and a movement towards more confrontational activism and a much more visible gay and lesbian presence (this is the time where the trans* people who were extremely central to some of the early movement work start to get pushed out of the narrative.) Although the 1970's were a time of increasing gay visibility, and to some extent, gay tolerance, there were concerns by the end of the decade that the movement had largely stalled. Gay writer Edmund White, in an essay called "Fantasia on the Seventies," wrote the following about the state of the movement: "Politically, the war will not take place...gay liberation as a militant program has turned out to be ineffectual, perhaps impossible." He goes on to discuss issues of class separation within the movement, and how relationships between gay and lesbian political movements had "collapsed." I find this to be a pretty accurate description of the state of the movement in the calm before the storm of AIDS.
Although work was absolutely being done during the pre-AIDS era - important political and community building work - it was just not on the same scale as what would come when the entire gay way of life seemed to be at stake. It's impossible to say just what would have happened, movement-wise, in the last 30 years if AIDS hadn't come into being (or hadn't affected the gay community.) However, I think it is pretty clear that the AIDS crisis kicked gay activism into high gear, and as such resulted in a lot of the groundwork that set the stage for later victories, including marriage.

In terms of ways in which AIDS impacted the movement in general, there are quite a few:

1. It forced people to come out.
   This is a big one. Although people talk about the 70's as the time when people started being "out" it was often a much more "qualified out" than what we consider being "out" today. A gay man might be out to a large social circle, have a lover, and be active in the gay scene, but not be out to his biological family or at work. When men started dying (often visibly) of AIDS it forced them to come out to their families, in the places of business, and to the world at large. More people coming out made gay and lesbian people more visible, which was a really massive part of the movement gaining ground.
2. Made confrontational activism "acceptable"
   By the late 1980's the situation was so dire that AIDS activists who had avoided confrontation activism out of a desire to look like "good gays" and a "deserving population" decided that they needed to take matters into their own hands. They had learned that the government was not going act on AIDS unless they were pushed into it. So confrontational AIDS activism, most notably ACT UP, was born. ACT UP was really massively influential. They played a central role in changing the set up and understanding of clinical trials, changing the CDC definition of AIDS to be more inclusive and allow more people to access AIDS services, speeding up the process of approving AIDS drugs, as well as providing education and giving AIDS a visible (beautiful, overtly queer/gay) face.
3. uniting gay men and lesbians
   This is perhaps one that we don't think of as much, but I think it is really critical, particularly in regards to the marriage movement. Although there was brief moment after Stonewall where gay men and lesbians attempted to work together on large scale movement work, they eventually settled into largely separate movements. Many lesbians became involved in AIDS work out of a desire to continue activist work; many brought with them extensive knowledge from feminist and/or lesbian-feminist organizing, as well as anti-war and women's health movements. Although there were certainly tensions in places like ACT UP between the two groups, AIDS definitely brought the two groups together.

Now to connect this all to marriage, and how the movement got there:
Marriage wasn't really even a political goal for the movement in the 70's and even most of the 80's. By the 90's two main things start to bring marriage into the forefront as a necessary and important goal. The experiences of gay men with AIDS showed the very real need for gay relationships to be legally recognized. It was by no means unusual for a gay man to be kicked out of his apartment after his lover (who's name was on the lease) passed away. A man's homophobic parents would be allowed to make end-of-life decisions for their son, while his partner (who had perhaps discussed end of life care with his lover) would be barred from doing so. A gay man's parents could prevent his partner or friends from attending or speaking at his funeral. Additionally, what came to be called the "lesbian baby boom" forced people to think more critically about what made a family, and how same-sex couples could create legal protection for their children.

So that was a lot of words to say that the AIDS crisis was pretty central to the movement's progress, and that the movement would have been unlikely to move at the same pace or in the same direction, had it not been for AIDS. I hope that was helpful!