1. The US Supreme Court's United States v. Windsor decision was a watershed moment for marriage equality.
2. Marriage equality isn't an issue for the coasts anymore. It's sweeping the heartland.
3. States are realizing that being gay-friendly is good for business.
4. Anti-gay policies in Russia and Uganda are reminding Americans of the costs of discrimination.
5. LGBT couples are making a difference by standing up and being counted.
While all of Mr. Lang's reasons are valid, and he explains them well, I think he misses two phenomena that make them all possible. The first has been taking place for about the past three decades or so. As more people "come out", still more people realize that they have family members, friends and co-workers who are not heterosexual or cisgender. Now, of course, some people reject or even commit physical violence upon important people in their lives who "come out". But we are seeing increasing numbers of people who realize that all of those LGBT people, after "coming out", will remain be the same people they loved or raised, or worked or hung out with. Very often, those people become supporters of marriage equality. I know: I have seen such transformations in my own life.
I don't know when the second phenomenon started, but I can say, with near-certainty, that it's more recent in origin than the first. More and more same-sex couples are raising children in places cities like Salt Lake City, Detroit and Memphis, and in states like Missisippi (which has the highest percentage of same-sex couples raising children). In such socially conservative places, people tend to delay "coming out"; many spend decades married to members of the opposite sex with whom they have children. In such places, people often couch their conservative religious and political beliefs in concern about "families" or the "welfare of children". To be fair, more than a few actually mean what they say. They may not appprove of same-sex parenting, or of same-sex love relationships in general, but they realize that a home with two moms or two dads who actually want (and, often, have the means) to raise a kid is better--and much less expensive for taxpayers--than foster care or any number of other alternatives.
As I have said in earlier posts, I would rather see the government's role in marriage end altogether, save for setting an age of consent. And, even though I am not against religion per se, I do not think that governments should vest churches or other religious institutions with the power to decide who's married and who isn't. But I don't expect what I've just described to come to pass, so I hope--and believe--that marriage equality will come to most, if not all, of the United States even sooner than I or many other people anticipated. Given the system we have, there is no saner alternative.