For some people, marriage between two people of the same sex insults their sensibilities. (and that is putting it mildly!) It is religiously wrong, because they have some document that proves that it is wrong. It trumps their sense of right and wrong. All the implications that can be thought of for why this should not be, they will find it!
There are so many boxes that have been created in our lives. Everything we do and all that we represent fits in those boxes. You can't be a cirle and fit in a square box, that doesn't work. You're going against the grain, against all that is natural, known and dare I say holy? In essense homosexuals do not fit the roles or the boxes that we have created in this life! Not in our lifetime, not in our backyards!
Am I being immoral because I have no objections to people of the same sex marrying each other? Some people do think that, I have no doubt about that. Same sex marriage is not an abomination of marriage in general, or against God as some like to quote. Same sex marriage does not make my own heterosexual marriage unimportant or less than what it is. What matters fundamentally is the right of each individual to choose the path that is their God-given right to do.
Although the legal papers now says that these people have the right to marry whomever they choose, they still do not have the legal rights, all the rights that a man and a woman in a marriage do. They won't be able to file taxes together, they won't be able to get all the benefits that a man and a woman in a marriage can from the government, if they need it, because although the law says they are allowed to marry, they are still not equal or legal in every aspects of their lives.
The article I have linked above, written by John Cloud, defines and clarify some of the things the California rulings does or does not do with the confusion to many about Gay marriage.
Marriage between homosexuals doesn't take away our rights as heterosexual individuals just because two men or two women seek to marry each other, but those who object gladly seek to take away what is a fundamental right of each person, their freedom!