I´ll define it by what I think matters the most. It´s purpose in our culture, tax benefits aside, is to solidify a relationship. Once you marry someone, It´s not between two people anymore but for everyone to recognize and accept that these two people love each other.
What it means to prohibit gay-marriage is simple. The message is: "your lifestyle is invalid to us. We may not put you into concentration camps, but you´re still less then human."
This kind of thinking, the stupidity, apathy and intollerance behind it affect and diminish all of us on every level and there´s no way we´ll overcome the bigger challenges ahead of us if we can´t even establish equality over something as basic (and presumably insignificant) as gender, race or sexuality. Ongoing globalization, overpopulation, climate change, the need for alternative energies and an impending food crisis if we keep using pesticides the way we do – all of these topics demand openess, engagement and a willingness to look deeper. The very same things that so many of us lack on the topic of homosexuality.
But we need this kind of energy. Desperately, so wherever we get in touch with it and get to train it like a muscle so it can grow stronger, that´s very positive and it gives me hope for our future.
The way I see humanity, we´ve matured quite a bit over the past century. It´s true that only a few of us are progressive on a personal level, but if you look at the bigger picture, progress is the only true constant about our species, be it in terms of technology, medicine or culture. Cultural change is a slow process, happening over generations and it´s clearly heading towards liberalism, no matter how much people are trying to hinder it´s course.
Compare all of humanity to a single person. A teenager who has only just begun to take responsibility. Every now and then he might relapse into a more infantile state due to his insecurities (as embodied by fundamentalists like the republicans party in the U.S.) but for the most part he has realized that fairytales aren´t real, that his actions have consequences and he has just begun to develop a real sense of empathy.
He might die in an accident before he gets to unfold his full potential or maybe he isn´t strong enough or he lacks the support it takes, but these are just negative possibilities. Likewise, even though nothing lasts forever, a society is just the same: If you give it a chance and create a fitting environment it can grow and mature into something beautiful. Just like individuals can.
Marriage equality is a huge step in creating such an environment.