Woman faces the death penalty if convicted on a 2011 murder charge as wife has been subpoenaed to testify against her

A same-sex couple in Kentucky have asked a judge to grant them the same protections given to heterosexual couples after a woman has been subpoenaed to testify against her wife.

Bobbie Jo Clary, 37, is accused of murdering a man in Louisville in October 2011 and could face the death penalty if convicted. The prosecution subpoenaed her partner, Geneva Case, 49, to testify against her.

The defense filed a motion to block the subpoena, citing spousal privilege, which protects married couples from having to testify against each other in most US jurisdictions. Clary and Case were joined in a civil union in Vermont in 2004, five years before the state legalised same-sex marriages.

The prosecution said it wants Case to testify because she allegedly heard Clary admit to the slaying and has other valuable information as a witness. Clary has admitted she was with the victim, George Murphy, 64, on the night of his death and says she used a hammer to defend herself after he sexually assaulted her.

Kentucky, a famously conservative state, though currently helmed by a Democratic governor, exempts spouses from testifying against each other in court. However, the state does not permit same-sex marriage and affirmed its opposition to the practice by amending the constitution in 2004 to define marriage as a union between a man and a woman.

"If they were a heterosexual married couple, there would be no question that Kentucky would acknowledge the privilege and not even have issued the subpoena," said Case's attorney Bryan Gatewood.

He said the basis of the argument to block the subpoena is the US Constitution's credit clause, which says that what's valid in one state should be valid in another. "It's a violation of equal protection not to do that," Gatewood said.

Kentucky recognizes common-law heterosexual marriages, which confer the rights of marriage to couples who have lived together for a certain period of time. Common-law marriages are only granted in a few states; Kentucky is not one of them.

"They will do it in that circumstance, but they are saying they will not do it for a gay couple couple no matter what – even if you're a gay couple married in Iowa or Minnesota – your marriage means nothing to the state of Kentucky – that's the crux of the commonwealth's position," Gatewood said.

Thirteen states now permit same-sex marriage, and the US supreme court struck down the Defense of Marriage Act in June, seemingly paving the way for a change in the ways which such standards are assessed by the courts.

Stacy Greive, assistant commonwealth attorney for Jefferson County, where the case is being held, told Reuters that Vermont's statutes distinguish between civil unions and marriage.

"Kentucky's marital privilege law does not give Ms Case the right not to testify in a murder trial," Greive said. "And the reason marital privilege does not apply to Ms Case in her relationship with the defendant is because it is our opinion and our belief that they do not have a marriage that is recognized under Kentucky law."

A ruling on the spousal privilege is expected by the end of the month. The murder trial is set to begin 30 August.