People should be able to believe whatever they want to believe. However, when those beliefs have a reasonable chance of hurting others, I don't think it's fair to let someone carry on with such beliefs. Everybody's got free choice. But when the choice is made to become destructive, people should be able to be stopped. You don't let a serial-killer on the loose, even if he believes it's his right, do you?
Not letting gay people marry is one of such beliefs, even if the repercussions are mild in this case. What if someone with the belief that gays shouldn't marry finds himself in the position of accepting or denying a couple standing before him? What if he has the power to deny their request, thereby hurting them?
And that's one of the mild examples I can think of. What if it's the parent of a gay child, who believes exorcism is a good thing? Thereby damaging the child? I think such freedoms are overrated; people blur the line between "think whatever you want to believe" and "oh, he's just a crackpot, hope he doesn't hurt someone". I don't think anybody should be free to have such destructive beliefs.
Accepting gay marriage is not destructive in any way. Denying gay marriage is. Since one is not destructive and the other one is, I can fairly say what's "good" and what's "bad". People tend to scream "you can't take my faith away from me, I've got free choice!". And I agree totally, even if those belief systems are destructive towards themselves. However, I draw the line where belief systems become destructive to others; free choice of beliefs should no longer be a right in those cases.
People should not have the right to damage others, especially their own children. Even if "their faith says them to".