Then what exactly is your justification?
There are a couple of reasons.
1. It violates what I've been taught to be right. (I understand this is a poor argument from a legal perspective, or a logical one.)
2. It goes against my religious convictions, which I hold and value very highly.
3. I feel it is important to uphold what we believe to be true and good and moral, so if there were a similar prop 8 type legislation in Utah, I would vote.
4. Through introspection and reasoning, I have determined that there is a natural aversion in me and others which it is possible to overcome (and should be overcome to facilitate understanding and tolerance). This aversion is not the result of cultural norms, familial teachings, religious tenets, predisposition to prejudice or anything else I can identify as a cause, except that my natural being is physically, emotionally and spiritually opposed to the idea of homosexuality.
5. There is still not conclusive scientific proof (that I am currently aware of) that homosexuality is completely out of the control of individuals. I will be doing some (unbiased) research over the next few months for a research paper citing specific studies, and as many as possible. This is for my own understanding of the issue, the work that has been done, and where I should go next in my actions or lack thereof in the context of homosexuality. I think we should legalize gay-marriage when it's proven that it's as inborn and unchangeable as race or sex.
6. I feel (honestly) that there is a more emotional drive to the gay-marriage end than a fight for "rights" or against "hate", etc. I feel that before we go changing the law, we should try to address the real source of the gay-anger. Once gay marriage is legal, will everything be terrific and resolute for homosexual people? I mean, is that the whole issue here?