1st Amendment: Congress shall make no law respecting an establishment of religion
Everyone giving a religious reason to why the law should be a certain way, in my opinion your argument is invalid.
Church does not make laws. They did at one point - and even through force gave the world the crusades...not exactly a highlight for humanity. The church (and religion in general) can be a beautiful thing (and in many cases, proves this daily - especially in Oklahoma), but when it comes to making/setting/influencing laws history has told us that is not smart. That is why those smarter than most decided we were to have a separation in Church and State.
Considering the idea of marriage as an official sanction derived from the Church/religion I have absolutely no problem in the Church standing behind their beliefs - stand behind them 365 days a year - you have the freedom to and that's what makes America great. Shout it from the Church rooftops that you want marriage to be a man/women union as it relates to your congregation. But from a legal standpoint, let America do its thing.
What this should be is a discussion on what the country deems acceptable as it relates to citizen equality and the benefits (taxes included) that should be given to those who choose to love each other and provide for each other throughout the rest of their lives - it does not take religion to tell us that is a wonderful thing.
Maybe the problem is in the wording. Maybe the government should stop calling it marriage - and continue to harp on branding it a Civil Union - as that actually is MORE of what it truly is from a legal standpoint.
I am not smart enough to break down the tax system of the United States government, but I do know that I am smart enough to know that if I can be recognized as being united in a Civil Union by my government (and receive financial benefits from my government) and my friends (and your fellow Americans) are not legally allowed to do so - that is wrong. I think it's time we officially stop making our neighbors drink from separate water fountains.