(Speaking from an American point of view)
As far as I see it, this is very similar to the black rights movement. People used to KNOW that black people shouldn't have rights equal to whites despite what the founding fathers wrote (all men are created equal). Now, we are at the verge of when people realize that marriage isn't just for men and women exclusively. Paradigms shift over time, and we can only hope that they shift with the use of reason. Why shouldn't two men or two women be able to share their lives and wealth together? There is no purpose to limit it. Nobody should dictate what an individual does with their life, so long as it doesn't harm others.

Hopefully this makes sense.