San Francisco resident Hak-Shing William Tam, a defendant in the case, discussed the letter to Chinese-Americans church groups during a legal deposition taped last month.
Tam wrote that legalizing same-sex marriage was part of a broader gay agenda.
"On their agenda list is: legalize having sex with children," states the letter, which also cautioned that "other states would fall into Satan's hands" if gays weren't stopped from marrying in California.
Lawyers for two same-sex couples introduced the footage to buttress their contention that Proposition 8 is unconstitutional because it was fueled by deep-seated animosity against gays.
During his deposition, Tam explained that he based his views on personal experience and a Web site that described a 1972 meeting of gay rights activists.
"My daughter told me her classmates chose to become lesbians and experiment with it after they noticed that same-sex marriage, they think it is a cool thing," Tam said. "They have some problem getting dates with boys, so same-sex marriage, since it is in the air, they think, 'Oh, why not try girls."'
Tam last week asked the judge hearing the case to remove him as a defendant because he feared the trial would generate publicity that could endanger him and his family. Chief U.S. District Judge Vaughn R. Walker has not yet ruled on the request. If it's granted, Tam could still be forced to testify.
David Thompson, a lawyer for Proposition 8 backers, told Walker that despite Tam's official role as a sponsor of the measure, Tam had nothing to do with the campaign and "is attempting to withdraw to avoid precisely this kind of focus on his individual views."
Thompson's characterization of Tam's role in the campaign contradicted Tam's description of his efforts when he urged the judge to allow him to be named an official party to the lawsuit last year.
"I dedicated the majority of my working hours between January 2008 and November 2008 toward qualifying Proposition 8 for the ballot and campaigning for its enactment," he wrote.
The trial under way here is the first in a federal court to examine the constitutionality of laws limiting marriage to a man and a woman. Regardless of the outcome, Walker's ruling is likely to make its way to the U.S. Supreme Court, where a decision could uphold or overturn bans on gay marriage nationwide.
On Wednesday, a psychologist from the University of California, Los Angeles, testified on behalf of two same-sex couples in the lawsuit that her research showed gay and straight couples "are indistinguishable" in terms of the stability of and satisfaction with their relationships.
Based on her own and other studies, Letitia Peplau concluded that nothing suggests that extending marriage to same-sex couples would cause marriage rates to fall and divorces to rise for heterosexuals.
"It is very hard for me to imagine you would have a happily married couple who would say, 'Gertrude, we have been married for 30 years, but I think we have to throw in the towel because Adam and Stewart down the block got married," Peplau said.
But she also acknowledged that her answers were not research-based because there has been no long-term study on the social impact of same-sex marriage.
During cross cross-examination, Proposition 8 lawyer Nicole Moss questioned Peplau about the children of such marriages. Gay marriage ban backers are trying to prove that states validly restrict marriage to a man and a woman to promote responsible parenthood.
"If your question is if two lesbians can accidentally, spontaneously impregnate each other: not to my knowledge," Peplau said.
The trial is scheduled to resume Thursday with testimony from an economist for the San Francisco city government on the financial costs to the city of not allowing gays to marry.