Not all Christians, and not even all Evangelical Christians, are opposed to same sex marriage. A moving account from an Evangelical Christian minister in Portland, Oregon whose take on the issue mirrors the values of the Christianity (and Boy Scout experiences) that I remember from my childhood reaffirm that.
While I have left Christianity, because I don't believe in God, I hold many of the values that I learned as a child from the perspective of a secular humanist. Those values place doing unto others as you would have them do to you and love higher up the authority scale than outdated Biblical pronouncements.
Unfortunately, in part because a lot of people who hold these values also doubt the metaphysical mysticism of Christianity, there aren't nearly as many actively practicing Christians like that left anymore. But, it is encouraging to see that there are still some Christians who know how to act like Christians left.