The intensifying fight over the marriage amendment took a contentious turn Thursday when the group pushing the measure released a television ad warning that same-sex marriage could be taught in school if the measure fails.

"Parents have a right to know that if marriage is redefined in Minnesota, same-sex marriage could be taught in public schools without prior notice, and parents could lose their right to opt their children out of such instruction," said John Helmberger, chairman of Minnesota for Marriage, the lead group pushing the measure.

The group trying to defeat the measure immediately blasted the ad, calling it inaccurate and misleading.

"It's clear that supporters of this amendment do not wish to debate the issue truly at hand: Whether it should be illegal to marry the person you love, and whether we should use our state Constitution to limit the freedom to marry," said Richard Carlbom, campaign manager for Minnesotans United for All Families. "Instead, supporters of the marriage amendment continue to try to mislead voters about what's at stake with this amendment."

The ad opens with a narrator saying: "If gay marriage happens here, schools could teach that boys could marry boys."

The ad features Massachusetts parents David and Tonia Parker.

"After Massachusetts redefined marriage, local schools taught it to children in second grade, including the school our son attended," David Parker says in the ad.

Parker goes on to say courts ruled that parents couldn't be told when the lessons would occur or pull their son out of class.

"If marriage is redefined in Minnesota, same-sex marriage could be taught in public schools, just as it was in Massachusetts," Tonia Parker says. "Don't make the same mistake and think gay marriage won't affect you."

Opponents point out the amendment voters will decide on Nov. 6 does not legalize gay marriage. State law already prohibits same-sex marriage. The amendment would sanction marriage as only the union between a man and a woman.

Anti-amendment forces say the other side is distorting and exaggerating the effect of the amendment to scare people into voting yes.

But Minnesota for Marriage spokeswoman Autumn Leva said that without constitutional protection, either courts or the Legislature could change the definition of marriage without direct input from voters.

Minnesotans United also argues that the ad misstates the nature of the Parkers' situation.

A year after Massachusetts legalized same-sex marriage in 2004, the Parkers' son was sent home with a "diversity book bag" that included the book, "Who's in a Family?" The book depicted different types of families, including single-parent families, interracial families and two same-sex families. The book concludes: "Who's in a family? The people who love you most!"

The Parkers demanded they be allowed to exempt their child from exposure to any books they found "morally repugnant." When school administrators rejected the request, the Parkers filed a lawsuit arguing the book was part of a public school effort to "indoctrinate young children" to accept same-sex relationships.

Massachusetts law requires parents to be notified of lesson plans involving sexuality and does give them the option of removing their child from the class. But courts sided with administrators, saying the book did not primarily involve sex education or human sexuality.

Leva said her group believes the ad to be "absolutely accurate."

Minnesota for Marriage has spent $750,000 to run the ad statewide, said Andy Parrish, deputy campaign manager. He said the group has several more ads in the works.

Baird Helgeson • 651-925-5044