It's undeniable that the right to marry a partner of one's choosing is guaranteed by United States law. It's part of life, liberty, and the pursuit of happiness. It's been argued that a ban on same sex marriage violates the equal protection clause. All people are created equal, and thus denying rights to a certain group of people is unconstitutional. If a man wants to marry a woman he can, but if a woman wants to marry a woman, she can't. This is discriminating on the basis of one's gender.
The first amendment states that "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof." It has been argued that since the only reason to ban gay marriage is a religious one, banning gay marriage would violate the first amendment by establishing a religion. However, such a ban would also violate the first amendment in another way. Since Unitarians and Episcopalians perform gay marriages, banning same sex marriage would prevent them from freely exercising their religion. Those pushing for a ban on gay marriage cite religion expression as their main reason, but ironically, a ban on gay marriage takes away the right to freely practice religion from certain denominations.