The United States is moving steadily toward accepting same-sex marriage. But is that trend inexorable?

One by one, states are legalizing gay marriage. Delaware, Minnesota, and Rhode Island joined the list this year. Hawaii and Illinois soon will bring the number to 16 states, plus the District of Columbia.

US Supreme Court decisions this year voiding part of the 1996 Defense of Marriage Act (DOMA) and effectively doing the same to California’s Prop. 8 ban on same-sex marriage no doubt have accelerated movement in that direction.

Oregon is likely to have the subject on a ballot measure next year; Michigan, Ohio, and Arizona may as well.

Until now, ballot measures on the subject have tended to affirm marriages under state law as limited to one man and one woman. But polls in many states – as they do nationally – now show majority approval of gay marriage, or at least opposition to state constitutional bans on same-sex marriage. That includes Pennsylvania, Indiana, Nevada, Ohio, Colorado, Arizona, Michigan, and Oregon, according to this Monitor survey of 11 gay marriage battleground states.

“The more people are winning, the more people are stepping up and wanting to become involved and move forward after,” Evan Wolfson, founder and president of Freedom to Marry, told Time. “The more we make it real – the more places gay people share in the freedom to marry – the more people see with their own eyes families helped and no one hurt.”

“They used to say you could only win in the coast, not in the heartland,” Mr. Wolfson said. “But we’ve won in Minnesota and Iowa. With Illinois, we have 37 percent of American people living in a freedom-to-marry state, including states in the heartland with more to come.”

The next federal court case could come in deeply conservative Idaho, where four couples (all women) are suing the state in federal court to challenge laws banning same-sex marriage and denying recognition to same-sex couples who married in other states.

Sue Latta and Traci Ehlers, Lori and Sharene Watsen, Shelia Robertson and Andrea Altmayer, and Amber Beierle and Rachael Robertson filed the lawsuit in Boise's US District Court on Friday.

"Idaho's exclusion of same-sex couples from marriage and refusal to respect existing marriages undermines the plaintiff couples' ability to achieve their life goals and dreams, disadvantages them financially, and denies them 'dignity and status of immense import,'" the women wrote in their lawsuit. "Further, they and their children are stigmatized and relegated to a second-class status by being barred from marriage."

In the Idaho lawsuit, the women note that they are allowed to file joint federal tax returns just as any married couple may, but that they are prohibited from joint tax filing status in Idaho, forcing them to file separately here. They also note that they lack the right in Idaho to make decisions for an ill or incapacitated spouse, the right to recognition as a legal parent, and a host of other rights and responsibilities otherwise afforded to married couples in the state.

The Hawaii House of Representatives passed a special session bill on Friday night legalizing gay marriage, setting up a final approval by the state Senate before it's sent to Gov. Neil Abercrombie for his signature. The Senate passed an earlier version last week.

"I commend the House of Representatives for taking this historic vote to move justice and equality forward," Gov. Abercrombie said in a statement after the House vote. "After more than 50 hours of public testimony from thousands of testifiers on both sides of the issue, evaluating dozens of amendments, and deliberating procedures through hours of floor debates, the House passed this significant bill, which directly creates a balance between marriage equity for same-sex couples and protects our First Amendment freedoms for religious organizations.”

This report includes material from the Associated Press.