Many have pointed out that marriage is a religious rite. As that's the case, IRS, state, municipal, military, employment, and any other secular, civil benefits that currently accrue to marriage shouldn't accrue to marriage at all. In fact, I should think that when they do, there's a probable violation of the separation of Church and State clause.
It's not an argument I've read before. This just came to me as one possible way forward: Let's let the religious fundamentalists control their fundamentalist religions, and enable the rest of us to control civil society in the USA. To that end, how about taking the equality emphasis off the rite of "marriage" and putting it on civil unions? It would work like this:
Provided ALL current and future secular, civil benefits that now accrue to marriage will then accrue to civil unions, let's push for equality in civil unions. Then, everyone who is civilly united will be entitled to all civil, secular benefits that now accrue to marriage. If they want the special religious benefits that accrue to marriage, they'll have to get married. The denominations will decide who can marry, and the Officially Godly People can fight that out among themselves and leave the rest of us in peace.