U.S. Federal Code Title 5 Part III says, "The President may prescribe rules which shall prohibit... discrimination because of marital status."
More than 1,000 US laws provide overt legal or financial benefits to married couples. In fact, unmarried individuals can pay over a million dollars more than their married counterparts for healthcare, taxes, insurance, etc, over a lifetime.
The system is unfair, particularly to single parents, as well as to all single people. Any and all tax laws favoring married couples over single individuals should be removed from the books.