Any married couple that earns more than 400 percent of the federal poverty level—that is $62,040—for a family of two earns too much for subsidies under Obamacare. "If you're over 400 percent of poverty, you're never eligible for premium" support, explains Gary Claxton, director of the Health Care Marketplace Project at the Kaiser Family Foundation.

But if that same couple lived together unmarried, they could earn up to $45,960 each—$91,920 total—and still be eligible for subsidies through the exchanges in New York state, where insurance is comparatively expensive and the state exchange was set up in such a way as to not provide lower rates for younger people. (Subsidy eligibility is calculated using a complicated formula involving income in relation to the poverty line, family size, and the price of plans offered through a state's marketplace.)

Nona and Aaron's 2012 income was higher than the 400 percent mark, but not by much. In New York City, that still doesn't take you very far for two people. If their most recent months of income are in the same range, they will get no help at all with buying insurance through the exchanges if and when they apply, according to the Kaiser Family Foundation and eHealth subsidy calculators. Premiums for the two for silver-level plans came in at $9,248 for the year.

But if they applied as unmarried individuals with something like their 2012 income, one of them would get at least $3,964 in subsidies toward the purchase of a plan, or possibly even be eligible for Medicaid, thanks to their uneven individual earnings that year. And if they fall below the 400 percent threshold, which Nona says they might this year, they could get substantial subsidies as a couple that are still worth less than what they'd be eligible for as individuals. These gaps are the marriage penalty.

Married people who are uninsured make up just a small fraction of the uninsured, for obvious reasons: It is easier to be insured if you have two potential pathways of getting there. Only 15.4 percent of married people were uninsured 2012, according to research from the Kaiser Family Foundation; the uninsurance rate for "single adults living together" was more than twice as high—33.4 percent.

That may be one reason the Obamacare subsides are more generous to single people and one- or two-parent families with children in the house than to couples who lack children. They were designed to help single moms and struggling middle-class families with children, not married creative-class millennials in pricey cities who have not yet settled into well-paid work, or barring that, work for a single employer.

Health insurance isn't the only place where there's a marriage penalty. The federal income tax also hits married couples with similar earnings harder than couples with one main breadwinner.