The fourteenth amendment states that, "No State shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States..."
Then explain why the government clearly abridges the privileges of married individuals? Gay or straight, marriage, civil union, or domestic partnership, all should be treated equal and fairly as the rest of us who are single. According to data from 2010 by the US Census Bureau, that is 43.6% of all adults over 18 years of age in America, not including a large portion of the armed forces where that number is over 50%.
You can hide behind the fact that the amendment is meant for states, though taxes clearly show they disregard it. Either way, "We hold that these truths to be self-evident, that all men are created equal..."