I had always intuitively considered marriage — not pair-bonding nor economic contracts, but marriage — as a silly, cultish, sentimental ritual: largely harmless but mostly ridiculous. By the same token, I had the same opinion of same-sex marriage.
But when I noticed the kinds of people who were opposing same-sex marriage, how and why, and what else they opposed (which included the values I hold the most dear, such as kindness, fairness and tolerance towards all) I realized that same-sex marriage (despite its silliness) was something to be resolutely defended against such opposition.
In much the same way, although I am, like most decent people, resolutely opposed to murder, including infanticide, pre- and post-partum, I noticed that the kinds of people who were opposing abortion were not vegans, defending the life of all sentient organisms, but again pretty much those same people who were against same-sex marriage (along with kindness and tolerance)