The difference between homophobia and heterosexism
Before now GLBT people have wanted to be tolerated - not criminalised, attacked, fired from their jobs etc for who they are.
Same sex marriage is different it's about more than tolerance. It's about same-gender loving people standing up and demanding that their they be treated in every way equally. It's about believing that in every way same-sex relationships are of equal value to different-sex relationships.
Some might say that opposition to same sex marriage is homophobic. It isn't. It's not about people having a fear or hatred of GLBT people. But it is about heterosexism - it is about believing that same-gender relationships are of an inferior status to different-sex ones.
There are plenty of people who are not homophobic, who would not wish any ill to queer people, who may think of themselves as quite liberal and open, but who nevertheless oppose marriage equality. There are people who think that tolerance is enough. And that (though they may not think of it this way) GLBT people should know their place. They are allowed to exist in society but they shouldn't claim that their relationships are equal.
But while not being homophobic this is heterosexist. Queer people (shock horror!) are not content to simply be tolerated as second class citizens. We are demanding full equal rights. We are saying that our relationships are of an equal value as anybody else's, and therefore should be treated the same under the law.
Same sex marriage is the last legal fight for GLBT people. There will still be homophobia in the world (as there is still racism) but society as a whole will be making the statement that homphobia and heterosexism is wrong. That's why marriage equality matters.