Employers’ standard practice of including legal spouses in health insurance is likely to place people in unmarried couples at a significant disadvantage for obtaining coverage. Data from married and unmarried couples in the Current Population Survey confirm that people with unmarried partners are two to three times more likely to lack health insurance than are people in married couples, even after controlling for factors that influence coverage. A requirement to provide the same benefits for partners as are provided to spouses would reduce the proportion of uninsured people in same-sex couples and different-sex couples by as much as 50%. We find no evidence of adverse selection. We predict that a typical employer offering domestic partner coverage will see a small increase in enrollment, ranging from 0.1% to 0.3% for same-sex partners and 1.3% to 2.1% for different-sex unmarried partners.