Gay Marriages on a Federal level needs to be allowed because of the United States Federal Constitution. Under the First Amendment: "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances."
If this is true, then laws regarding marriage, need to be unbiased. Marriage is both an institution of law and religion. On the law side of things, gay marriages need to be allowed because if not... that infringes on the First Amendment. Marriages, whether accepted by the Church or not, has no bearing on gays who want to be afforded the same laws and protections as a straight couple.