If bigots oppose gay marriage so vehemently, it must be because marriage is a defining institution for them; gays will never be fully accepted until they can marry and adopt, like anyone else.
The Stonewall riots were a key moment for gay people. Throughout modern history, gays had thought of themselves as something like a mental illness or maybe a sin or a crime. Gay liberation allowed us to make the leap to being a 'minority group,' which made life much easier.
Originally I was opposed to gay assimilation and targeted gay marriage as just another effort on the part of gays to resemble their straight neighbours.
First, I was opposed to gay marriage because it seemed like one more way that gays were wanting to assimilate. When I realized the Christian right was so opposed to it, as well as tyrannical governments in Africa and Russia, I thought, 'It must be a good thing to fight for.'
AIDS had won gays sympathy; they no longer seemed the privileged brats that the general populace had resented in the 1970s.