That bypasses the issue: the left wants to continue redefining the English language so that our laws no longer mean what they used to mean. Once they succeed, the Constitution will actually be meaningless. We must not allow them to redefine the word "marriage."
I think we have an interest in lowering taxes for everyone, but our traditional family unit is especially vulnerable to taxation. That's why we recognize traditional families with social security and tax exemptions. If the government can't rely on a given county's marriage proofs, then we shouldn't trust them enough to pay spousal benefits to people trying to use those documents to prove their elegibility.
About equal protection: this should be debated and cast aside. We have equal protection for the unions blessed by our civilization for 4,000 years. Men and women who marry are equally protected by our laws that say men and women who are not relatives can be married.