Those rights are expanding as legally married gay couples relocate to states that don’t allow same-sex marriage, forcing courts, legislatures and employers to deal with the resulting issues of custody, divorce, inheritance and end-of-life decisions.

The adoption ruling in Maine had the effect of granting parental rights to same-sex couples. By the time the Legislature adjourns for the summer, experts expect Maine to become the fifth state to legalize same-sex marriage — 11 years after voters banned it.

In California, federal judges have twice overruled decisions by the federal government to deny healthcare coverage to gay employees’ legal spouses, teeing up a constitutional challenge to the 1996 Defense of Marriage Act, which forbids federal benefits for same-sex couples.

Same-sex marriage is legal in Connecticut, Iowa, Vermont and Massachusetts, which began the trend five years ago. (Iowa issued its first marriage licenses April 27, a few weeks after its Supreme Court gave approval; weddings in Vermont will begin in September.) Within a year, Maine, New Hampshire, New Jersey and New York will probably follow suit, say sexual orientation scholars at the UCLA School of Law’s Williams Institute; New Hampshire’s Senate approved a same-sex marriage bill Wednesday.

And as more same-sex couples wed in places where it is legal, the administrative fallout in other states is expected to keep expanding.

“The courts are going to have to wrestle with these issues as more and more states make it possible for people to marry,” said Toni Broaddus, executive director of the San Francisco-based Equality Federation. “People don’t stay in the same state for their whole lives anymore, so the courts in states without marriage equality are going to have to address these issues.”

The recent moves in New England and the heartland to legalize gay marriage appeared to reinvigorate campaigns for passage of same-sex marriage bills in Maine, Maryland and Hawaii. Rights advocates predict the tide will eventually sweep even into some of the 30-plus states that have passed laws or constitutional amendments defining marriage as between a man and a woman.

“A body of law is emerging because it has no choice. Cases have been filed and they have to be decided one way or another,” said Joseph Milizio, a Long Island lawyer specializing in gay and lesbian representation.

The legal developments allow people to become comfortable with “the fact that gay marriage is going to be recognized in many different aspects, even in states that don’t allow it,” said Milizio, whose firm recently secured the first dissolution of a same-sex marriage in New York.

In the workplace, proponents of extending spousal rights such as healthcare benefits and life insurance to same-sex couples have succeeded by challenging employment practices that discriminate on the basis of sexual orientation. Seven states, including California, now guarantee full equality to same-sex couples — another incremental advance that is lamented by opponents.

 See Same-sex marriages gradually gain legal ground