Legalizing gay marriage will have many effects on others besides the gay community. The principles we stand on as a country affect the entire country. If as a country we accept homosexual marriage as an acceptable behavior for all, we are as a country sponsoring that behavior, just as we currently sponsor traditional marriage. What is legal helps define our culture, and it is a stretch to say that culture will not affect anyone but those who were originally sponsoring gay marriage. It is also difficult to say that legalizing gay marriage will not influence future laws and policies regarding the education process, adoption requirements, or who knows what else.
Perhaps this is a classic "slippery slope" argument. Maybe the gay community will be happy with being able have gay marriage sponsored by the state, and they will pursue no further action. Perhaps they will not try to have gay marriage taught as acceptable to our kids, and perhaps they will feel that adoption agencies should not be required to place kids in homosexual homes. Perhaps the gay community will then feel that their actions are no longer looked upon with disdain, so they will no longer try to spread their moral beliefs via policy and legal changes to anyone but those within their own community.
Even if the only effect of overturning Prop 8 is to influence culture, culture affects the people who live in it. We can argue as to whether those cultural changes are good or bad or should be legislated or not, but we can't say that one group's actions will have no effect on anyone who interacts with them in the same society.