The opposition to gay marriage is not based on rational or moral considerations. If it were, convincing reasons could be given in support of the position. Three reasons in particular are typically offered, but none is compelling. All fail to make the case.
1. Homosexuality and gay marriage are contrary to nature. Natural law is supposed to provide a rational basis for morality that all competent reasoners can recognize. A good theory, but it doesn't work,since fully rational persons come to divergent conclusions. In the argument over female suffrage, e. g., both the proponents and the opponents appealed to "self-evident" natural law. The test of universality fails. Problems abound:
A. Natural law is always what somebody says it is. There is no universal agreement today about what natural law teaches about gay marriage or many other subjects. What is called natural law regarding homosexuality is nothing more than a cultural belief or individual conviction given transcendent authority by locating it in the very moral structure of the universe and the mind of God. Some natural law claims may indeed reflect an objective order of truth and value, but we cannot be sure of that, and we cannot know for sure which claims,if any, in fact do so. The claim that reason rightly employed will produce universal claims cannot bear close scrutiny, since natural law changes on particular issues with cultural consciousness and interpreters past and present disagree about what natural law requires.
B. Natural law has been used in the past to defend what nearly everybody now recognizes to be evil. Slavery, the denial of the vote to women, and the segregation of the races were said in times past to be in accordance with natural law. Opinions about what natural law requires on particular subjects changes with changing cultural values. The only way to get universal agreement is to stay at some very high level of generality, e. g., that good is to be done and evil avoided. Duh!
Hence, the argument from natural law fails. It is no more than somebody's current opinion.
2. Homosexuality is condemned by Scripture. So it is in Leviticus 20:13 and perhaps in Romans 1:26-27. The problem here is that those who find compelling authority in particular passages must also affirm that gay men be killed like the Leviticus passage says, that men can sell their daughters into slavery (Exodus 21:7)and that disobedient sons should be stoned to death (Deut. 21:18-21. Slavery is nowhere condemned in the Bible, and is everywhere assumed. Those who condemn homosexuality use the supportive texts that are available but conveniently ignore other passages that would require them to do things that are abhorrent to them and most everybody else. There is a lot of bad morality in specific passages. Biblical morality must be judged by what is highest and best in its witness. Paul's best advice was given in Romans 13:8-10 and I Corinthians 6:12. Love of neighbor fulfills the law, and all things are permitted that are not harmful and that are helpful. Responsible same-sex love harms no one; it is helpful for those who find fulfillment in it; and it is the fulfillment of the law of love.
3. Gay sex and marriage are harmful to society. No good reason can be given to justify this claim. Heterosexual marriage could go on as always. No one would be harmed, and gay people would be greatly benefited. What are the bad consequences that would follow? Who would be hurt? What would be lost that is worth preserving? I have yet to see a persuasive argument that individuals or society as a whole would harmed. What we need is a change of attitude, thinking, and law.
Instead of persuasive reasons, what we get are ungrounded assertions,dogmatic pronouncements, taboo, visceral reactions based on upbringing, cultural traditions, bad religion, and the like, none of which will stand rational scrutiny in light of the highest moral principles of reason and Scripture.
For a more detailed version of these ideas, see: