Though none of the recent regulatory updates regarding legally married same-sex couples applies to domestic partnerships or civil unions, the rule changes could substantially affect mid-market employers' decisions to cover such arrangements under their benefits plans.

As a result of the U.S. Supreme Court's partial strike down of the Defense of Marriage Act in June, the Obama administration has extended eligibility to married same-sex couples for rights and benefits guaranteed under the U.S. Tax Code, the Employee Retirement Income Security Act, and the Family and Medical Leave Act.

But those rule changes apply only to couples that have been legally wed in a state, territory or foreign country that permits same-sex marriage, meaning employers will remain at liberty to decide for themselves whether to offer health care coverage and other benefits to their employees' domestic partners.

More than 52% of midsize employers — defined as companies with 500 to 4,999 full-time employees — polled in August by the International Foundation of Employee Benefit Plans said they cover domestic partnerships and civil unions under their employee benefit plans. After the Supreme Court's ruling, 75.4% of those employers said they intend to continue to offer benefits to domestic partners and civil unions at least for now.

“We are seeing a lot of midsize employers, especially in states that permit same-sex marriage, saying that they are planning to eventually do away with coverage for domestic partners, just not for this coming plan year,” said James Harmon, president of the employee benefits division at Cleveland-based Dawson Consulting Group, a subsidiary of AssuredPartners Inc.

A crucial factor in many mid-market employers' decisions about whether they will continue offering benefits to same-sex domestic partners likely will be the extent to which they also intend to cover opposite-sex domestic partners. Fifty-seven percent of mid-market employers polled in the recent survey said they do not cover opposite-sex domestic partners, and only one in five of those employers said they were considering doing so as a result of the Supreme Court's decision.

In many cases, employers choose to offer benefits only to same-sex domestic partners, as opposite-sex couples can marry under both state and federal law.

After the expansion of same-sex couples' rights under the U.S. Tax Code, ERISA and FMLA, benefits experts said employers who continue to exclude opposite-sex couples from coverage could be accused of discrimination, at least in the 13 states — plus the District of Columbia — that have legalized same-sex marriage.

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“Now that same-sex domestic partners have the option to get married, it will be a lot harder for an employer to justify limiting their policy to cover same-sex couples only,” said Todd Solomon, a Chicago-based partner at law firm McDermott Will & Emery L.L.P.

Employers operating in multiple states also may expose themselves to potential civil lawsuits by restricting their coverage of domestic partners only to states that do not recognize same-sex marriage.

“A large percentage of our client base is made up of multistate employers, and I think it would be hard for them to offer domestic partner coverage to some employees in some states and not to others,” said Don Garlitz, executive director for exchange solutions at the Chicago-based benefit consultancy bswift Inc. “If they're going to make a decision as an employer to offer coverage, they're either going to have to go all in or all out.”

Employers operating solely in states that do not recognize same-sex marriage may not face the same level of exposure to legal action for continuing only to cover same-sex domestic partners. Even if the option to marry is technically available in other states, these couples would still be treated disparately under their home state's tax law.

Regardless, experts said the safest course treats as equal all individuals situated similarly under state and/or federal laws.

“From a compliance point of view, you want to treat everyone the same way,” Mr. Harmon said. “You might still have some latitude in the states that don't permit same-sex marriage, because you'll have that differential in the treatment of the two groups under state law. But at least in the states that do allow same-sex marriages, we would advise our clients to have one policy and one set of rules, regardless of the gender of the partners.”