While I am in favor of legalizing same sex marriage, I do think that Malcolm's argument is not mathematically flawed. Mr. K. already mentioned estate taxes. Let's run an example. Adam and Steve want to get married but the law prevents them from doing so. Adam dies (in 2013) and leaves $10,000,000 to Steve. If they are not married in the eyes of the state, then the federal government government will step in a snag a share[1] of that money. However, if Adam and Steve are married, then the government doesn't step in to grab that money because money left to a widow is exempt from estate taxes.
If the government collects less money and keeps spending constant, then either the deficit goes up or someone is going to have to pay more federal taxes.
One might then say, the amount of money lost in that manner is so negligible that it would be silly to even consider it. I would disagree with that assertion. If one truly believes a tax to be unjust, then making that tax very, very, very small does not make it a just tax.
Note: there are other taxing scenarios and situations which would involve gay couples keeping more of their income. I do think it is necessary to spell each one out in detail in order to validate the claim that if gay marriage were legalized, the federal government would collect less money.
Disclaimer: Malcolm, I apologize if I have mischaracterized any part of your position.
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[1] upwards of $ 1,500,000 if I am reading the tax tables correctly.