Susan Jacoby writes abou the necessity of and the political courage required to approach civil issues from a firmly secluristic position in this Op-Ed piece in the NYTimes: One Nation, Under Secularism (free but registration req'd).
She refutes the idea that the founders weren't secularly motivated and that abolition of slavery, civil rights for racial minorities and women's rights were successfully fought by organized religion.
"...Abraham Lincoln, whose spiritual beliefs were so elusive that both atheists and the devoutly religious have tried to claim him as their own, spoke eloquently on this point during his long period of deliberation before issuing the Emancipation Proclamation.
"I am approached with the most opposite opinions and advice, and that by religious men, who are equally certain that they represent the divine will," he told a group of ministers in September 1862. "I hope it will not be irreverent for me to say that if it is probable that God would reveal his will to others, on a point so connected with my duty, it might be supposed that he would reveal it directly to me. . . . These are not, however, the days of miracles. . . . I must study the plain, physical facts of the case, ascertain what is possible, and learn what appears to be wise and right."
Today, many voters, of many religious beliefs, might well be receptive to a candidate who forthrightly declares that his vision of social justice will be determined by the "plain, physical facts of the case" on humanity's green and fragile earth. But that would take an inspirational leader who glories in the nation's secular heritage and is not afraid to say so. ..."
I think this is the foundation of the case to be made for civil marriage equality for LGBT folk.