Beth McKinnon, left, and partner Ann Carper at their Georgetown home. The couple might get hit hard by taxes if marrying and filing jointly increases their tax brackets. (Michael S. Williamson/The Washington Post)

Refreshed by a late September vacation in Canada, Beth Mc­Kinnon and Ann Carper were ready for the job ahead — figuring out whether it makes sense to get married as a result of the Supreme Court’s summer ruling on the Defense of Marriage Act.

The women have been together since 1998, and they have already tied the knot in two ways, a civil union in 2001 in Vermont and a domestic partnership filed in the District, where they live. “We feel married in the eyes of our friends and family,” Carper said.

But now that they could also be married in the eyes of the federal government, that might change the equation. And it’s an equation with many parts. The ruling that struck down the federal law known as DOMA and subsequent regulations issued by the Internal Revenue Service and other federal agencies mean that same-sex spouses are now eligible for many benefits they were previously denied.

But they also might get hit hard by taxes if marrying and filing jointly increases their tax brackets. That’s what McKinnon and Carper hope to sort through in the next few weeks.

If they get married, “it would really be for financial reasons, as unromantic as it sounds,” said Carper, sitting at a kitchen table next to a window that overlooks their flower and vegetable gardens.

Not everyone takes that analytic approach.

For many couples, the romantic impulse took over, but when financial planner Tom Tillery’s clients called saying they were rushing off to where it was legal to marry, he said he held up a stop sign.

“For our gay and lesbian clients, it was heartbreaking,” said Tillery, who has clients around the country, recounting a conversation with a client who called excitedly after the ruling to say she was flying to New York to marry her partner.

Tillery and others recommend what Carper and McKinnon are doing — taking the time required to sort out the financial implications. After the ruling “the biggest complication was laying myself in front of the train of my clients wanting to run off and get married,” said Tillery, whose firm, Paraklete Financial, is based in Kennesaw, Ga.

The federal government has been relatively quick to issue rulings based on the decision. The Office of Personnel Management and the Pentagon extended benefits to the spouses married where same-sex unions are legal, while the Department of Homeland Security recognized such marriages as the basis for immigration. The Defense Department also extended 10 days’ leave to members of the military planning to marry, to make it easier to travel to where same-sex marriages may be legally celebrated.

On Aug. 29, the IRS followed in those agencies’ footsteps, ruling that same-sex couples legally married in jurisdictions that recognize their marriages will be treated as married for federal tax purposes.

“This ruling also assures legally married same-sex couples that they can move freely throughout the country knowing that their federal filing status will not change,” Treasury Secretary Jacob Lew said.

On Sept. 4, Attorney General Eric H. Holder Jr. advised congressional leaders that President Obama had directed the Department of Veterans Affairs to provide benefits to spouses in same-sex marriages despite a federal statute that limits spousal benefits to spouses of the opposite sex.

What about Social Security?

One major question to be answered is what the Social Security Administration will do. It bases spousal benefits on a couple’s place of residence, rather than where the marriage was performed. Even so, when it comes to Social Security, experts are advising individuals to apply for spousal and survivor benefits as soon as they can, because the benefits, once approved, are retroactive to the date of application. Social Security has encouraged not just same-sex married couples but also couples in other legally recognized relationships such as civil unions and registered domestic partnerships to apply.

According to Patricia A. Cain, a tax professor at Santa Clara University and a national expert on sexuality and the law, the IRS ruling was timely because many taxpayers had postponed filing their 2012 taxes by asking for extensions, and the decision came through before the Oct. 15 deadline.

Among other complications when it comes to federal income taxes, couples will have to decide whether to amend separately filed individual returns for the past three years, based on whether they would pay more. As a rule of thumb, two individuals who both earn relatively high incomes would likely be worse off by amending returns, because of the possibility of a higher tax rate applied to the pooled earnings, while a couple with one high-earning individual and another who is lower income might benefit from bracket-lowering dilution.

On the other hand, a dual high-earning couple might still want to marry to qualify for the marital exemption or deduction from estate taxes. While federal estate taxes apply only to estates in excess of $5.25 million, 21 states and the District have estate taxes, most of which kick in at $1 million. Although the ruling on DOMA doesn’t affect those laws, it’s a consideration for some couples in deciding whether to get married.

You could try timing it to get the best of both worlds, said Laurie Kane Burkhardt, a financial planner for Modera Wealth Management, which has offices in Massachusetts and New Jersey. “If you’re going to stay with someone for the rest of your life and have enough assets that your partner might end up paying the estate tax, you may not want to get married until later in life,” she said. Of course, the risk is that “you can’t time dying.”

And, in the case of a couple with a large earnings discrepancy, if the low-earning individual receives any assistance based on income, such as the earned income tax credit or federal subsidies to help reduce the cost of insurance bought in the recently opened health-care exchanges, filing jointly might reduce or eliminate those benefits.

Then there is the issue of taxes paid before 2013. Taxpayers are allowed to refile taxes and claim refunds going back three years.

One other major consideration couples should take into account is what happens in the event of divorce, said Susan Sommer, director of constitutional litigation for Lambda Legal. “One thing for people to remember is that, while there is no residency requirement to get married to a same-sex spouse [in jurisdictions that recognize them], there generally are residency requirements to get divorced.” Of the places where same-sex marriages are permitted, the District, California, Delaware, Minnesota and Vermont allow couples who were married there but live elsewhere to divorce under their laws, although Vermont has some limits.

Federal worker benefits

Although clearing up the financial calculus will take time and probably result in additional litigation, there are some couples for whom getting married is much more clearly a winning strategy, said Jerry Cannizzaro, a financial planner with the Reston-based firm Ticknor Atherton & Associates. They are couples in which one member is a federal worker. “Federal government employees are in a great situation” because federal benefits are so good, he said.

“The federal government has guaranteed health insurance for life,” Cannizzaro said. The surviving spouse is covered even after a retired federal employee dies, he said. The surviving spouse would also be entitled to a percentage of the worker’s retirement annuity.

Cannizzaro’s clients include Carper, who is a federal employee, and McKinnon, who is a retired Social Security manager. McKinnon is thinking through how it would work if she went back and opted for survivor benefits that she didn’t take when she retired. Her pension would be reduced, but Carper would receive benefits if she outlived McKinnon.

That’s one of the issues that the women, who both describe themselves as practical and frugal, will be discussing with Cannizzaro — and with their accountant and an estate attorney.

While waiting for the details to be sorted out after the DOMA decision, activists are moving the fight for marriage equality into the states. “We’re still living in a patchwork country where rights are far from complete,” said Lambda Legal’s Sommer. “We’re in a state of incremental equality. It’s far better than where we were a few months ago, but we still have a way to go.”

A guide to the issues raised by the DOMA ruling was produced by the American Civil Liberties Union, the Center for American Progress, Family Equality Council, Freedom to Marry, Gay & Lesbian Advocates & Defenders, the Human Rights Campaign, Immigration Equality, Lambda Legal, the National Center for Lesbian Rights, the National Gay and Lesbian Task Force and OutServe-SLDN.