The right to marry is about a lot more than tax equality. But in dollars and cents, the right to file joint tax returns is nothing compared to what happens when a relationship dissolves. When a married couple splits, there’s no limit on the money or property the two can transfer tax-free between them. See IRS Publication 504, Divorced or Separated Individuals.
Marriages end, and even our Byzantine tax code recognizes that it is unfair to tax either party. Dividing the assets is not a sale for tax purposes. That means no income tax to either party.
What’s more, there’s no gift tax either, unless the spouse receiving assets isn’t a U.S. citizen. Compare this tax-favored divorce to what happens when an unwed couple splits. It’s all taxed.