Opponents of California's ban on gay marriage are using an anti-gay activist's claims against him in a landmark gay marriage trial taking place in San Francisco.

Lawyers working to have Proposition 8 -- California's ban on gay marriage -- overturned as unconstitutional played a videotape of William Tam, a leading proponent of Proposition 8, arguing that allowing gay marriage would lead to the legalization of sex with children.

Opponents of Prop 8, who launched the constitutional challenge, are using Tam's comments "to buttress the contention that Proposition 8 is unconstitutional because it was fueled by deep-seated animosity against gays," reports the Associated Press.

Earlier this week, as the trial got underway, Tam asked to have himself removed from the case, in which he was acting as a defendant. (Governor Arnold Schwarzenegger and state Attorney General Jerry Brown, who are named as the defendants in the lawsuit, declined to defend Prop 8, leaving the court to appoint Prop 8 backers as defendants.)

Tam, one of the principal mobilizers of the movement to pass Proposition 8, had argued that he wanted out because the publicity surrounding the case was endangering him and his family.

''In the past I have received threats on my life, had my property vandalized and am recognized on the streets due to my association with Proposition 8,'' Tam told the court. ''Now that the subject lawsuit is going to trial, I fear I will get more publicity, be more recognizable and that the risk of harm to me and my family will increase.''

But blogger Timothy Kincaid, writing at Box Turtle Bulletin, didn't believe that explanation, and argued that Tam's decision to withdraw had everything to do with his prior statements that gay marriage would lead to legalized child rape.

[Tam's] concerns about being recognized didn’t seem to have dissuaded Bill Tam from giving interviews and making videos and participating in debates during the campaign. And the worrisome issues didn’t give him enough concern to keep him from petitioning the court in May 2009 to be added as a defendant. And Tam provides no instances since May in which anyone recognizing him has been anything other than “friendly”.

Kincaid pointed to news reports that Tam had sent a letter to his supporters in which he outlined the "disastrous consequences" of gay marriage to California.

"One by one, other states would fall into Satan's hands," Tam reportedly wrote. "Every child, when growing up, would fantasize marrying someone of the same sex. More children would become homosexuals."

Lawyers challenging Prop 8 played a video of a deposition Tam gave the court in December during the testimony of Yale law professor George Chauncey, who is testifying in favor of gay marriage. In the deposition, Tam describes the letter he sent out linking gay marriage to child sex.

The San Jose Mercury-News live blog of the trial reports that Chauncey said Tam's letter "is consistent in tone with a much larger history of anti-gay rhetoric."

Lawyers for Prop 8 supporters argued that homosexuals do not need to be protected from discrimination because the US has become more tolerant towards gays and other minorities.

"The bottom line is that there has been a significant shift in public opinion toward acceptance of gay rights, correct?" Prop 8 lawyer David Thompson asked, as quoted at the Mercury-News blog.

"There has been a shift in public opinion and growing support for gay people, and gay people continue to encounter enormous hostility," Chauncey replied.