It Doesn't Matter What the Bible says About Homosexuality
This is not a theocracy. The bible should not in any way inform our laws. Nor should the Qur'an. Nor the Torah. Nor the Bahagavad Gita. Nor the Jain Agamas. Nor the Book of Mormon. Nor any other religious text. Period. Taking the argument to the bible misses the point.
Every individual has a right to practice religion, or not practice religion, in whatever manner they wish; as long as doing so does not infringe on the same rights of another. When we restrict an individual or group's rights based on the religion of any other individual or group, we deny that person the constitutional protection to freedom of religion. It's really that simple.
Allowing same-sex couples to marry does not restrict anyone else's right to practice their own religion. It doesn't prevent a man and woman to marry in their own church. It doesn't, or shouldn't, force a church who disagrees with same-sex marriage to perform the wedding ceremony.
On the other hand, restricting same-sex couples from marrying DOES infringe on someone's right to freedom of religion. It takes the religious belief of one and imposes it on another. It is unconstitutional period.
There is no need to show that it is okay within a religion, because that doesn't matter. We are Americans. We don't have to justify our actions to any religion.