Have a read of this. This page is a fairly comprehensive summary as to the Christian arguements agaisnt gay marriage. I have summaried the main points below as a TL;DR version.
Same-sex Marriage, Gay, Homosexual, and Lesbian Unions
1. Effects on Moral and Religious Standards:
- The Bible (as well as many other religious texts) condemn homosexuality, and by extention, renounce gay marriage.
- By accepting gay marriage, it presents homosexuality as good and wholesome.
2. Effects on Traditional Marriage:
- Firstly the Bible (as well as other religious texts) authorise marriage as a relationship strictly between a heterosexual couple.
- Accepting gay marriage would be promoting a perverted relationship to the dignified status currently held by honourable marriage, thus tarnishing the wholesomeness of traditional marriage.
3. Effects on Children:
- Part of traditional marriage involves the companship of a mother and father caring for their children. Gay couples are unable to concieve children.
- Homosexual marriages deprive children of the right to being raised by mother and father.
4. Effects on Religious Organisations:
- The Church is responsible to preach the truth by God and oppose moral evils.
- Legislation and encouraging of gay marriage will pressure religious organisations to accept homosexuality.
5. Effects on Government and Education:
- The church and government have separate roles, but both church and government share a common duty to promote good morals and discourage evil conduct.
- Legislating gay marriage would put the government in the position of justifying evil.