If you live in California and need help explaining to someone why you're voting YES on proposition 8, there is a great 8 page article here by Christian apologist Dr. Frank Turek. One of the things I like about his article is that he shows how it is possible to make a reasonable argument to protect traditional marriage and oppose the legalization of homosexual marriage without appealing to the Bible. I, as you know, am not opposed to appealing to the Bible. But if you are talking to a person who has a low view of Scripture, this may not be a very effective approach.
Below is a brief overview of his argument (his full length article fleshes out each of the points in more depth).
1. Traditional marriage is beneficial to the public welfare.
2. Homosexual behavior is destructive to the public welfare.
3. The law is a great teacher; it encourages or discourages behavior and attitudes.
4. Legalization of homosexual marriage would encourage more homosexual behavior, which is inherently destructive. It also would weaken the perceived importance of traditional marriage and its parenting role, thereby resulting in further destruction of the family and society itself.
5. The law should endorse behaviors that are beneficial and restrain (or certainly not endorse) behaviors that are destructive.
6. Therefore, the law should endorse traditional marriage and it should restrain (or certainly not endorse) homosexual marriage.
Click here for full article.