Three Utahns filed a federal lawsuit Monday seeking to overturn a century-old ban on polygamy, claiming the prohibition against taking plural spouses violates their constitutional rights.

The trio — a married couple and the man's would-be second wife — applied for a marriage license last month through the Salt Lake County Clerk's Office but was denied on the grounds that plural marriages are illegal in Utah.

The lawsuit alleges the denial violates the plaintiffs' First Amendment rights, saying they were simply trying to practice their "sincere and deeply held religious beliefs."

"It's an important issue. . . . Polygamy has defined the history of Utah," said Brian Barnard, the trio's attorney. "The fact of the matter is that people engage in polygamy for legitimate religious reasons, and it should be protected."

The Church of Jesus Christ of Latter-day Saints sanctioned polygamist marriages until 1890, when it abandoned the practice, and has since excommunicated church members who take plural wives. There is no official tally, but some experts say as many as 100,000 people from a variety of religious backgrounds practice polygamy, primarily in the West.

Polygamy is banned in the Utah Constitution. Bigamy, being married to more than one person at a time, is outlawed and is a third-degree felony punishable by up to five years in prison.

According to the lawsuit, those who practice polygamy as a basic tenet of their religion have a constitutional right to do so.

"This is a clean-cut legal issue: Can consenting adults with full knowledge practicing deeply held religious beliefs practice polygamy?" Barnard said. "I want to give (Utah Attorney General) Mark Shurtleff an opportunity to tell me and to tell the world why polygamy practiced for religious reasons by consenting adults should be a crime."

Shurtleff was unapologetic Monday for his hard-line stance against polygamy and polygamy-related crimes. Since his 2000 election, Shurtleff's office has been involved in several polygamy prosecutions, including high-profile polygamists Tom Green and Rodney Holm.

"My job is to enforce the law; I didn't make the law," he said. "(Bigamy) is a third-degree felony in this state, so if (Barnard) disagrees with it he ought to be spending his time up here lobbying the Legislature to change the law."

Until then, Shurtleff said he will continue to enforce the law as written. The Attorney General's Office is not named in the complaint, but it often becomes involved in defending constitutional challenges to state laws.

The suit seeks the reversal of an 1879 U.S. Supreme Court decision upholding a federal ban on polygamy. In the case, George Reynolds, personal secretary to early LDS leader Brigham Young, was prosecuted for practicing polygamy.