The unprecedented expansion of marriage rights for gay and lesbian couples in the past year has exerted added pressure on employers to review the health care benefits they extend to employees' same-sex spouses and domestic partners.

A year ago, the U.S. Supreme Court struck down a core provision of the 1996 Defense of Marriage Act that excluded married same-sex couples from recognition under federal law.

The high court's June 2013 ruling in United States v. Windsor touched off a series of revisions and updates to federal statutes under which married gay and lesbian couples were previously denied the same rights and protections afforded to opposite-sex married couples, including preferential tax treatment, eligibility rights and other spousal benefits via an employer-sponsored health insurance plan.

“The Windsor decision does, I think, make things simpler for employers in the long term,” said Todd Solomon, a Chicago-based partner at McDermott Will & Emery L.L.P. “In the short term, though, there's obviously been a lot of upheaval and changes made in the last year.”

Additionally, the Supreme Court's verdict has lent considerable strength to the dozens of pending lawsuits seeking to overturn the remaining state-level statutory and constitutional prohibitions against gay marriage, several of which have already been struck down in federal and state district courts.

And while the seismic shifts in the legal landscape for married same-sex couples has only a limited bearing on employee health benefits provided to domestic partners from a compliance perspective, the changes have raised substantive strategic questions about employers' intent to continue providing domestic partner benefits for same-sex or opposite-sex couples.

Where employers' health benefit programs are concerned, the broadest and most immediate regulatory change resulting from the ruling was the extension of equal recognition and protection for legally wed same-sex couples under the U.S. Tax Code, regardless of their state of residence.

“That's probably the closest the federal government can get to a national definition of spouses that includes married same-sex couples without taking the next (legislative) steps,” said Rachel Cutler Shim, a Chicago-based tax, benefits and wealth planning attorney at Reed Smith L.L.P.

That rule change, which went into effect Sept. 16, 2013, likely provided a considerable measure of administrative relief for the 61% of employers that already offered health care coverage to their employees' same-sex spouses at the time of the Supreme Court's ruling, according to an August 2013 survey by the International Foundation of Employee Benefit Plans.

With married same-sex and opposite-sex couples on equal tax footing with the IRS, employers no longer are required to pay federal payroll taxes for medical, dental and vision insurance provided to same-sex spouses, nor are gay and lesbian employees required to pay federal taxes on contributions made toward their spouses' coverage.

The U.S. Department of the Treasury and Internal Revenue Service further ruled that employers and employees can seek refunds for taxes paid on same-sex spousal health care coverage in plan years between 2010 and 2013.

Under the agencies' revenue ruling, employers cannot collect refunds or credits for employment or Social Security and Medicare tax overpayments until they have repaid or reimbursed over-collected taxes to their employees. It remains unclear as to whether employers are required to fulfill employees for corrected earnings statements if the company has no intention of seeking a refund for itself.

“In my experience, almost every employer is forgoing the refund opportunity because it's just not that much money and it's a very complicated process,” Mr. Solomon said. “It's not clear to me whether that employer has an obligation to issue a corrected W-2 to employees if asked for one, and I think a lot of employers will be asked as people work through this.”

The Supreme Court's partial overturn of DOMA also led the IRS to revise its rules regarding cafeteria health plans. Prior to the ruling, employers could not allow married gay and lesbian employees to elect accident and health care coverage on a pretax basis under a cafeteria plan unless the spouse also qualified as a tax dependent.

The IRS said in a December 2013 guidance document that employees would be permitted to enroll their same-sex spouses for pretax coverage under a cafeteria plan retroactively to the beginning of whichever plan year includes the date of the Windsor ruling.

Additionally, employees now are permitted to use tax-favored spending accounts — including health, dependent care, adoption assistance and flexible spending accounts — to reimburse covered expenses incurred by their same-sex spouses or their spouses' dependents, as long as those expenses were incurred within the plan year that includes the date of the Supreme Court's decision or the date of their marriage, whichever is later.

Legally speaking, the Windsor verdict and the subsequent regulatory updates have done relatively little to change the extent to which employers are required by federal law to offer health care benefits to their employees' same-sex spouses.

Under the Consolidated Omnibus Budget Reconciliation Act, or COBRA, employers must now offer continual health care coverage to same-sex spouses as a result of the Supreme Court's ruling. Same-sex spouses also have been extended special enrollment and nondiscrimination rights under the Health Insurance Portability and Accountability Act.

Additionally, the U.S. Health and Human Services Department clarified in March that the nondiscrimination provisions of the Patient Protection and Affordable Care Act prohibit health insurers from offering group health care plans that include disparate coverage for opposite-sex and same-sex spouses.

However, employers' obligations to provide spousal health care coverage remain governed primarily according to their funding structure and only secondarily by the states in which they operate.

Employers that provide fully insured health care plans in the 19 states and District of Columbia that have legalized same-sex marriage are generally required to offer spousal coverage to same-sex and opposite-sex couples in equal measure.

Meanwhile, despite the U.S. Department of Labor's expanded definitions of “spouse” and “marriage” under the Employee Retirement Income Security Act, employers that self-insure their group health benefit plans still have discretionary authority to define eligibility for spousal coverage as they see fit, regardless of state laws.

Experts have warned that although federal law does not necessarily require self-insured employers to extend their health plan to gay and lesbian spouses, the broader protections now afforded to married same-sex couples could lead to civil litigation under federal and/or state nondiscrimination laws.

Continuing to offer spousal health benefits to opposite-sex married couples without extending equal benefits to married gay and lesbian couples could expose self-insured employers to accusations of gender-based discrimination under Title VII of the Civil Rights Act of 1964, particularly employers located in states that have legalized same-sex marriage, experts say.

“That's becoming more and more likely, given some of the litigation and opinions that have come out so far (on states' same-sex marriage bans),” said Jay Kirschbaum, St. Louis-based national human capital practice leader at Willis North America Inc. “It might depend on the state that the employer operates in, but I would think that if there's no protection for making the distinction between opposite-sex spouses and same-sex spouses at the state level, then accordingly there would be no protection for that kind of distinction at the federal level.”