Look at the example of Prohibition. No one thought this could pass, and yet it did. How? For several generations, being sober, "dry", and favorably inclined to temperance organizations served as a litmus test for political office in about two-thirds of the country. Maybe 40% of the country was sincerely "dry," but politicians representing the wets did not want to run against that issue so they side-stepped it by trying to play both sides of the issues. Before long, politicians of the country were far more "dry" than the people they represented.
When the 18th amendment was finally proposed - as a wartime measure - no one expected it to pass but, sure enough, the Prohibition people had the votes and it passed pretty quickly.
This is why I'm concerned about the Janus-faced positions of John Kerry and John Edwards. On one hand, I can understand why they want to take the issue off the table: they should easily beat Bush on the remaining issues. On the other hand, their refusal to be strong on the issue adds some potentially long-term credibility to the position on the other side. If all presidential candidates have to say "I'm against gay marriage," soon most governor candidates and Senate candidates will do the same thing. A half-hearted position does not balance a full-hearted position, and if we let enough half-hearted Democrats win our primaries, before long the conservatives will have their votes for their bigoted amendments. If we appease the bigots, and let opposition to gay marriage become the "dry" position of the next generation, then we will have a bigot amendment eventually. That's how they will pass the FMA.
Also, this is another reason I like John Lewis as Kerry's VP. Kerry can dodge the issue by letting Lewis bulldog for the truth.