I probably differ with many of my Republican and Conservative friends with respect to my support of Gay marriage. Here's my logic on the issue:
The only legitimate use of force is done in retaliation to stop the individual(s) who initiated the use of force - the Individual Right of Self Defense. Government's only legitimate role is to protect the individual rights of it's citizens. It is the Individual Right of Self Defense that authorizes government to act as the agent of our protector in cases where rights are being violated, to defend the individual and stop (or punish) the offender(s).
As Jefferson once said, "But it does me no injury for my neighbor to say there are twenty gods or no God. It neither picks my pocket nor breaks my leg." So long as the beliefs, or actions, of another individual do not violate the rights of others, then there is no justification for using the force of government to stop his actions (bans on gay marriage) or otherwise punish him for his beliefs. To do so would be immoral.
What consenting adults choose to do with their lives is none of my business, and by extension, it is also none of government's business. While most "gay" rights activists would likely agree with that statement, even the most ardent supporter of "gay" rights would - through some feet of mental gymnastics - suddenly claim it IS government's business with regards to polygamy (which I also support, as it does not violate anyone's rights).
If you can admit that two gays marrying one another does not violate your rights but still believe you have a right, and by extension government has a right, to ban social behavior simply because you find it repugnant, then anything you do that someone else finds repugnant (such as keeping the fruits of your labor or, worse yet, accumulating wealth) is also fair game. By supporting a system that places demands or bans on people whose actions are not violating the rights of anyone, you have no standing to oppose whatever demands they feel necessary to place on you.