First of all, we are talking about rights.
If you choose to go the Christian route, then we are all created in God's image. He knows each one of our hearts and minds perfectly, and He loves us. God does not hate. Let me say that again. God does not hate. In addition, we are all made as equals and should therefore have equal rights. There is no question here. The loving God supports love, and the fair God supports equality. It is only natural to conclude that, under God, we should be equal in marriage--the promise of love.
If you choose to accept the fact that, here in the United States of America, we stand for separation of church and state, then what is it that stands between the two halves of a same-sex couple? Is it more than that between halves of an opposite-sex couple? Is it less? The answer is no on both accounts. Love is love, and the genders of those who feel it do not affect its validity.
If you are among the individuals who oppose same-sex marriage, then I ask you to imagine a mass of people telling you that you and your fiance cannot be husband and wife. Imagine state courts ruling that you cannot be married, purely because there are people who oppose the unity between you and the person whom you love. Just as it is right for a man and woman to marry, it is right for a man and a man or a woman and a woman to marry. Just as no harm will be done by the marriage of an adult man and an adult woman, no harm will be done by the marriage of adult men and the marriage of adult women.
Constitutionally, ethically, sincerely, it is wrong to deny marriage of same-sex couples. There is nothing lawful about withholding the rights of a people. There is nothing lawful about disregarding equality and freedom in regards to love--arguably the most beautiful and important concept in the world.