(I am currently on an Ethics Bowl team and have compiled an Ethical defense of Same-Sex Marriage)
So the main issue that is often brought up in discussions of Same-Sex Marriage is the right of the individual versus morality. The claim is that we must, as public citizens, decide between the two. Do we side with the right of the individual and support Same-Sex Marriage or do we side with morality and oppose it? I attest that the two are, in fact, not mutually exclusive. Same-Sex Marriage is ethical by any conventional standard. It is ethical under Consequentialism, ethical under Deontological theory, ethical under virtue ethics, and ethical under Pragmatic Ethics. It is also succeeds in non-maleficence.
Consequentialism, incidentally my favorite of all the theories, contends that the way to decide if an action is moral or not is to examine the consequences. The consequences of two consenting adults getting married is clearly not immoral. No one is hurt and some(not many but some) are helped. The couple is happy, the couple's family is happy, and the couple's friends are happy. Again, none are unhappy, under a utilitarian argument(Utilitarianism=the greatest good for the greatest number, it's a popular offshoot of consequentialism) Same-Sex Marriage is clearly ethical.
Deontological Ethics is the opposite of consequentialism. It contends that the way to decide if an action is moral is to examine the intentions. They are often moral absolutists who believe that some things are immoral no matter what is the consequence. If we look at the intentions of, again, two consenting adults entering into matrimony we can assume that, in this day and age, the intentions are positive. The intentions are positive thus, under this theory, the action is ethical. So, Same-Sex Marriage is ethical.
Virtue Ethics says that the way to judge the morality of an action is to assess the morality of the individual. There is no way of assessing the morality of every individual entering into any form of marriage. Thus, we can say, that Same-Sex Marriage is just as ethical as any other form of marriage.
Pragmatic Ethics contend that it is society that is moral, not an individual. Under pragmatic ethics it is believed that ethical standards evolve and what is right in one age is not necessarily right in another. Societal standards have changed and it is now socially acceptable to be openly homosexual. Thus, if we take it that moral standards evolve and change, homosexuality could very well have been wrong when the Bible was written but still be right now because the moral standards have been revised. Thus, Same-Sex Marriage could very well be ethical under Pragmatic Ethics.
Non-maleficence is the simple concept of "Do No Harm." If two consenting adults get married then no one is harmed. Thus, non-maleficence is achieved.
As I have shown, Same-Sex Marriage is ethical under every modern applicable ethical theory. Thus, we can conclude, that Same-Sex Marriage is ethical and should be supported.