To be clear we need: Democrats against gay marriage
The reason why the millions of left leaning voters who oppose gay rights in general and gay marriage in specific don't speak up for their values:
1. The Hollywood elite and MSM have perpetuated a cultural Nazi-like discipline that has prevented reasonable discussion of moderate and liberal opposition to gay rights. And the voters are taking their cues from this.
2. Most people want to be liked. The radical left has convinced those who support gay rights to be loud and vocal. This is cowered many people into silence because they don't want to engage in a "divisive" issue. So the end result is that on the one side is "Loud and Proud" and on the other side is a combination of "this is my personal opinion" or silence.
3. A concerted effort by rich homosexuals to infiltrate unrelated groups and make them an accepting environment to their radical causes. Whether it's Environmental groups, Labor Unions, Peace movements, etc. While there may not be an overt talk about the radical gay rights agenda, they make sure there is neutrality and when given a "below the radar" choice, they choose to attack our society's values in whatever they can get away with. An example of this would be to have a person most would identify as a homosexual because of typical stereotypes as your spokesperson (see Mitt Romney campaign). Even though he was supposedly going to deal with foreign policy, somewhere along the line he would have influence and use it to hurt America. This was with the Republicans, and they barely got him to leave. Romney is so weak he couldn't stand up to the Right. Democrats would accept him willingly. That's the problem.
4. The pro gay rights crowd is well-versed in explaining their positions. While many people who believe in traditional values either are basing their views on their faith (which is not proper to speak about in public?!) or they aren't the intellectual type trying to advance a cogent logical argument. So we end up with one side on a mission to convert you to their thinking, and the majority of people just trying to get through their day. Mismatch.
5. The most vocal supporters of traditional values are right wing Republicans who bother most Democratic leaning voters such as myself. It is reflexive to think if "they're against it" then I should be for it. If they support tax cuts for the rich, against cleaning up the enviroment, against affirmative action, etc. then it is difficult to grant they are right on this issue. In addition they attack the President and Democrats so visciously that many voters naturally defend our side. But we need to do what's right for America. Politics is complicated. Each party is a coalition, and we shouldn't let our views be minimized for the sake of comity.
All of the above can change with a concentrated effort. It starts with having the courage to simply state your opinion with your friends and in public.