At midnight on Dec. 29, same-sex couples in Maine became eligible for marriage licenses, and some lined up on a frigid night to be among the first to file the paperwork that made their households into families in the eyes the law.

At that same stroke of 12, other couples experienced the same kind of change, even if they didn’t budge from their warm homes: They were the same-sex couples who were legally married in other states who suddenly became legally married in Maine when the law went into effect.

Nothing the Supreme Court can do in the pair of same-sex marriage cases on which it hears oral arguments this week can take away what those Maine couples achieved last year, but the court still could have a profoundly positive impact on their lives.

While marriage is largely a matter of state law, the federal ban on same-sex marriage means that no state can create full marriage equality. No matter what a state’s courts, Legislature or, in Maine’s case, its people decide, same-sex couples have different benefits and obligations under federal law than heterosexual married couples and so are not being treated equally.

That’s why the court should declare the federal same-sex-marriage ban, known as the Defense of Marriage Act, unconstitutional and let progress on this issue continue.

The friend of the court brief filed by attorneys general in states in which same-sex marriage is legal — including Maine Attorney General Janet Mills – declares that the state has a strong interest in marriage. Married couples create stable households, are more financially secure and take on legal obligations to care for each other and their children that relieve the state of expensive burdens.

The state has no interest in whether these couples can produce biological children, the brief reads, and states have always issued marriage licenses to couples who cannot or choose not to have children.

But as Edith Windsor of New York discovered, the woman to whom she was married became a legal stranger when she died as far as the IRS is concerned. Windsor, the plaintiff in one of the cases the court is hearing, received a $300,000 inheritance tax bill that would not have been sent to another widow.

Maine couples married here will encounter the same double standard. They can’t file joint federal tax returns or be treated as family members and survivors for key benefits. The Supreme Court should not let this unequal treatment stand.

A strong ruling for marriage equality would remove one legal impediment for these families, but not all. Just as some couples married in other states became legally married in Maine the moment the law took effect, those same couples would again become unmarried if they move or were transferred to another state that does not recognize their union.

Going from married to unmarried as they move from state to state is something other couples never have to deal with, and it is something that will have to change if full equality is ever going to be achieved.

But striking down DOMA would be a good next step in this long struggle.