Actual answer: If you live in an urban area, or are under the age of 30, you probably have someone close to you who is openly gay. The idea that they still cannot get married in certain areas, and that there is one major party that would probably consider rolling back some of the rights they've recently gained if they ever had the opportunity, can be a major rallying point.
There are many young people I know that have no interest in politics, except when it comes to gay marriage.
Very well said.
It's a big deal because it is unconstitutional for consenting adults and taxpaying citizens in a minority to be denied equal rights under the law all in the name of "muh Bible says gays are gross and will go to hell." I laugh at some of the asinine slippery slope arguments on the "traditional marriage" side like that it opens the door to polygamy and pedophiles marrying children and people marrying their cats and dogs and plants. When you just want to be treated as equals and not as second-class citizens and you have to confront this bigotry on a daily basis, it is a very big deal.