New York is on the verge of becoming the sixth state to legalize gay marriage — and the third to do so through legislation rather than litigation. But that's just one battle for pro-gay activists. In every other state to pass same-sex marriage, legalization has put opponents back on offense in courts and statehouses, with mixed results.

Recent history has shown two paths to follow: either the law quickly settles in to the status quo, as it seemed to do in Vermont, Connecticut, and Massachusetts; or the law faces challenge after challenge, becoming a perennial election issue.

Consider California and Maine, two states that once allowed gay marriage but have since walked back from their progressive stances. In California, same-sex marriage became legal following a state Supreme Court ruling, while in Maine it was a product of legislation. In both instances, opponents were able to get a ballot referendum on gay marriage and voters repealed it.

Other attempts to turn back the clock have proven less successful.

New Hampshire passed a gay marriage law in 2009, and conservative state legislators have tried to use the same legislative process to repeal it. Several bills have been introduced since legalization, some aiming for simple repeal, others hoping to amend the state constitution. The latter would require three-fifths approval in both houses of the legislature, as well as two-thirds approval from voters. With recent polls showing 62 percent of state voters against repeal, a constitutional amendment looks unlikely anytime soon.

In Washington D.C., opponents have attempted to go through both the legislature and the courts with little to show. Catholic Bishop Harry Jackson first attempted to hold a referendum on the City Council's passage of a same-sex marriage law, but the measure was blocked by the D.C. Board of Elections and Ethics. Jackson then sued the board, and the lawsuit wound up in the D.C. Court of Appeals and was dismissed summarily on the grounds that such a referendum would violate the city's Human Rights Act — the same justification given by the City Council for not allowing the ballot measure in the first place.

Connecticut, Iowa and Massachusetts are the only states currently allowing gay marriage that achieved this end through the judicial branch. Their Supreme Courts ruled that denying marriage to homosexuals was a violation of each state's constitution.

In 2007, Massachusetts successfully defeated a proposed constitutional amendment barring same-sex marriage post-court ruling. Earlier this year, the Iowa House of Representatives passed a constitutional amendment outlawing gay marriage, but the Senate hasn't followed suit. If they do, voters would still have to approve the amendment at the ballot box, which could take years.

It seems Vermont and Connecticut are the only states where repeal efforts have largely ceased. Take, for example, Connecticut and the state's Catholic Public Affairs Conference, which hasn't updated their same-sex marriage action website with new legislation since 2009.

If New York legalizes gay marriage — a big "if" at this point — the same arsenal of constitutional amendments and repeal bills and calls for referendum will be at opponents' disposal.