Some ideas are unthinkable, then they become thinkable. This happens all the time. I suspect that for the vast majority of people in this country, same-sex marriage was unthinkable right up to the moment they had to start thinking about it. They had never seen it, they had no history of thought about it, no experience with it. This happened to me around the fact of my being gay. I couldn’t think that I had rights. Every time I thought about my being gay, I thought about the stigmas we carried—criminals, mentally ill, sinful—but Stonewall said gays have rights. This was a whole different story.
I was forced into thinking about these matters by an article on the death of Frank Kameny, the founder of Mattachine Washington, and a participant in the Stonewall Riots and, it seems, in every significant gay rights action for decades after. The article is by David Carter, who has his own claim to our gratitude, since he is the author of the book-length study of the Stonewall Riots, Stonewall. Kameny had been a cartographer with the Army Map Service and in 1957 was fired for being gay. Kameny sued and took his case to the Supreme Court. He wrote a classic statement of the case for gay rights under the constitution, but, according to Carter, “in 1961, the Supreme Court was not ready to hear this analysis, and it did not take the case.” Carter quotes Barbara Gittings, who was also there at the beginning of Mattachine, saying, “before I met Frank … I had a very inchoate idea of how we could solve our problems. … Frank came along and he had this very strong, very definite philosophy, and it crystallized my thinking. ‘Well, yes, of course. If you take the position that Frank has taken, then you get a very clear view of what you have to do, and you don’t have to fumble around anymore.’ “
How does an idea become thinkable? 
One of the important questions of our time is just this one. Even before the governments could think about marriage and the military and adoption and the rest of them, majorities have formed to demand these changes. How did this happen? What did we do?