Imagine that the law was changed tomorrow. Instead of the law defining marriage as being a union between a man and a woman, the law changed its definition of marriage to being a union between two adults. The rules regarding everything else—age, degree of consanguinity, and so on—remain as they are now. The only change is that now two people of the same sex can get married, not just two people of the opposite sex.
Now, ask yourself this question: who is actually harmed by this?
Every existing marriage continues as it was before. Nobody’s relationship is harmed. Nobody’s family is harmed. No religious group is harmed: those religions that had been allowed to perform marriages on behalf of the state (as long as the couple meets the religion’s requirements for marriage) will still be able to perform marriages on behalf of the state (as long as the couple meets the religion’s requirements for marriage).
Sure, some people may feel uncomfortable at the thought, but am I harmed because you make me feel uncomfortable? True, there will be some people who hold a religious objection to same-sex marriage, but not allowing me to impose my religious views on you is not harming me. Yes, it might become harder to discriminate against same-sex couples, but making it more difficult for me to be prejudiced is not harming me.
So, who is harmed by same-sex marriage?