Thursday's warning came days after Romney said he would enforce a law dating back to 1913 that prohibits couples from out of state to marry in Massachusetts if their marriage would be void in their home state.
Same-sex marriage is set to become legal in Massachusetts on May 17 following a landmark ruling by the state's highest court last year stating that depriving gays and lesbians of the right to marry was unconstitutional.
"It is our view that same-sex marriage is not permitted under the laws of any other state in the nation," Romney, a Republican, said in a letter to governors and attorneys general of the other 49 states.
"Unless we receive an authoritative statement to the contrary from either you or your representative, the Commonwealth of Massachusetts will not issue a Massachusetts marriage license to same-sex couples from your state," he wrote.
His decision to invoke the 1913 law evoked outrage from gay rights advocates and confusion from town and city clerks, who told Reuters they have never enforced the statute in the past.
Mary Bonauto, an attorney for the gay rights group GLAD said on April 25, "Massachusetts has no legal justification at all for catering to discriminatory laws of other states, particularly where it has itself already declared that barring same-sex marriages is unconstitutional."
She also argued that "the governor's painting with far too broad a brush in his interpretation." According to Bonauto, this specific legal language could only apply to a limited number of states based on their current marriage policies.
Thirty-eight states have passed some form of a Defense of Marriage Act, which defines marriage as a union between a man and woman, either as a statute or constitutional amendment in response to the possibility of gay marriages. However, 19 of those states do not specify that a marriage performed in another state would be "void," according to a report in the Boston Globe.
In three other states -- New Hampshire, Vermont and Wyoming -- the marriage statutes include heterosexual gender references, Reuters reported. Seven states -- Connecticut, New Jersey, New Mexico, New York, Oregon, Rhode Island and Wisconsin -- have statues that do not reference gender but also do not affirm gay marriage.
The final state, Maryland, has not passed a Defense of Marriage Act but has another statute defining marriage as a union between a man and a woman.
It remains unclear whether the clerks who will be on the front lines of the gay marriage issue will comply with Romney's demands.
Judith St. Croix, vice president of the Massachusetts Town Clerks Association, said the group's members were awaiting gay marriage training sessions scheduled to begin next week.
"We don't have any answers until we have training," she told Reuters, adding that to start enforcing the 1913 law just for gay couples would constitute discrimination.
As the state prepares to begin marrying gay couples, the state's legislature last month gave preliminary approval to a constitutional ban on gay marriage, which would also create civil unions if approved by voters in November 2006.