SAN FRANCISCO — Lawyers for two same-sex couples challenging California’s ban on same-sex marriage plan to wrap up their case Friday following the incendiary testimony of a proponent who said he thinks gays are more likely to be pedophiles and that allowing them to wed would lead to the legalization of polygamy and incest.

Hak-Shing William Tam of San Francisco spent five hours testifying Thursday as a hostile plaintiffs’ witness to prove that bias toward gays fueled the 2008 campaign to pass the voter-approved measure, known as Proposition 8.

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Tam, who was one of five individuals who signed on as official proponents of the ban and whose names appeared alongside ballot arguments for Proposition 8, acknowledged that he subscribes to beliefs about an alleged link between homosexuality and pedophilia posted on the Internet by a Chinese-American Christian group for which he serves as secretary.

“Do you believe that homosexuals are 12 times more likely to molest children?” attorney David Boies asked.

“Yeah, based on the different literature that I have read,” Tam replied.

Earlier in the trial, a Cambridge University professor testified that there is no evidence to suggest that gays are more likely to molest children than heterosexuals. Boies pressed Tam to cite books, articles or authors he had read to substantiate the views, but Tam said he could not remember specifics.