To me it's very clear that gay marriage should be allowed. But maybe not for the reasons many people think.
If it were just a question of a "celebration of love" or "a holy contract between a man, a woman, and God", I honestly wouldn't care either way.
But that's not what marriage is. At least, not anymore. Government decided to get involved. In the present day world, marriage is a contract between two people and the government. There are tons of benefits of being married; actual, tangible benefits. This website has an enormous list of the benefits of marriage. Especially amusing is the quote "if you are in a domestic partnership or civil union... none of the benefits of marriage under federal law will apply to you, because the federal government does not recognize these same-sex relationships". Also, "...you may not file joint federal income tax returns with your partner... and other federal benefits, such as Social Security death benefits and COBRA continuation insurance coverage, may not apply".
This isn't a question about religion anymore. It stopped being about religion the moment government benefits came into the equation. It's about the fact that government has decided to endow couples certain special privileges, and as everyone is created equal, everyone should be entitled to these same privileges.
Either that, or eliminate every single privilege enjoyed by heterosexual married couples. Whatever floats your boat.