Marriage has absolutely dick to do with religion anymore. Marriage is a state-run institution that affords married couples certain rights and privileges that ensure the steadfastness of their union and protects it from state interference. That is all.
Perhaps the most important part of marriage is that married couples enjoy the right of spousal privilege in legal proceedings. One spouse may not be compelled by the State to testify against the other, nor reveal the contents of privileged communications, for example. That's huge, and it is something that gay couples in states that do not recognize or allow gay marriages do not have. Any two people who are committed to each other should have this right of privilege.
What States really ought to do is just remove the word "Marriage" from all of their books and only offer civil unions, to any couple who desires to have one. If you want to have a religious marriage, get one in a Church, and follow your religion's rules, but leave the rest of us out of it. Religious folk do not have the right to force the rest of us to follow their religion, and in fact our Constitution guarantees that gay couples are free from state-sanctioned discriminatory religious beliefs.