Let's dig a little deeper. If there is no reason (i.e. no logical reason) but a religious to be against same-sex marriage, why don't you support it?
I hope you don't mind if I, as someone who also doesn't support it, answers.
There are two answers to this question. The first is that I don't believe that marriage should be an institution that is controlled and regulated by the government. I also don't believe that marriage should afford you any privileges that are special and unique to the marriage agreement that are not also available to everyone by other similar agreements.
The second answer is that based on my belief that government shouldn't be involved in marriage, the marriage process is entirely a religious one or personal one.
The fact of the matter is that we all have different traditions and ceremonies associated with marriage and we all apply our own significance and meaning to the ceremony. That being said, a marriage between two people of the same sex cannot, by any religious beliefs I subscribe to, happen.
It's not that I think they should be illegal, law should not even be written concerning it as I see it as a religious or, if you're not religious, something that is done in accordance with one's personal beliefs to sanctify/beautify/glorify/etc the union between two people.
I don't believe that marriage belongs in the domain of law, but in the domain of personal behavior and beliefs. If one feels they need to be married, it is because of some personal significance that they apply to it and nothing more. For me, the significance I give to a marriage between a man and woman simply
cannot be applied to two people of the same sex. Not because of bias or discrimination, but because of the definition of marriage that I have and the very nature of the significance I give to it.