Who say so? That is exactly the problem. Some people think it evolved and some don't. Are people willing to go to war because of this? It is interesting. The American Civil War was because of slavery and never again it was accepted world-wide so it might happen again.

Some people think it's evolved and some don't, but that's a very good reason to get the Government out of the business of regulating what is a marriage and what isn't - otherwise you're forcing one social group's beliefs on another group.

My getting married - according to my definition of married - doesn't affect you in any way, shape, or form, so why would the Government prevent me from doing so?

I have interacted with people online who seemed to genuinely think that the ability of a same-sex couple, that they have never met and never will, to get married would deeply affect the value of the ongoing relationship they had with their opposite-sex spouse.

I never did get to the bottom of how that was supposed to work.