WELLESLEY - Congressman Barney Frank and Republican challenger Earl Sholley traded a somewhat testy exchange here Sunday over gay marriage.

Frank, Sholley and Susan Allen, another challenger of Frank's, traded blows in an hour-long debate at Wellesley College. In one of the final questions posed by the moderator, the candidates were asked for their position on gay marriage and if judges should have the authority to determine whether gay marriage should be legal.

Sholley said he strongly opposes gay marriage and that he supports letting people vote for a constitutional amendment banning it.

"I believe having gay marriage diminishes the value of the institution of marriage," he said. "I believe it's a sacred pact between a man and a woman and it is so ordained by God."

Sholley, who repeatedly referred to himself as a Christian, also said that gay marriage is not a civil rights issue because being gay is a choice.

"This isn't a civil rights issue," Sholley said. "This I has nothing to do with environment. This has nothing to do with genes. There is no gay gene. This has to do with choice. In fact, there is a lot of evidence to indicate...I believe that it's obsessive compulsive behavior it's been estimated that 80 percent of homosexuals were sexually abused as children. I think we should be helping these people."

Sholley's response was greeted by both boos and cheers from the crowd.

Frank, however, was quick to disagree with Sholley.

"I hope it won't be ungracious to reject Mr. Sholley's offer to help me overcome the nonexistent child abuse," the Newton Democrat said, gathering his own applause from the crowd.

"Same-sex marriage is of no concern to anyone who does not choose to enter into one," Frank went on. "The notion that it weakens the institution of marriage is nonsense. We've had same-sex marriage in Massachusetts for four years; there has been no negative effect."

As far as putting the issue to a vote, Frank noted that in both the 2004 and 2006 Massachusetts statewide elections, gay marriage was a central issue on which every candidate was asked to take a side. And, Frank said, Bay State voters elected a legislature "that voted by about 77 percent in favor of same-sex marriage."

"I believe the voters do know what they are doing and I believe the voters showed their choice," he said.

Allen, who was very critical of Frank at other points in the debate, was on his side of this issue.

"I am pro choice when it comes to marriage," the Democrat-cum-Independent said. "Each person has a right to marry whomever they most dearly love."

Frank also objected to Sholley's statements generally.

"I have to say, I do not understand what motivates people," Frank said, "[to be] concerned with interfering with the happiness of others."