SAN FRANCISCO — A leading proponent of Proposition 8 found himself on trial Thursday inside the boundaries of the historic federal court trial here to decide the legality of California’s ban on same-sex marriage.

During an afternoon of scathing inquiry from prominent plaintiffs’ attorney David Boies, William Tam, a controversial figure in the Proposition 8 movement, was confronted repeatedly on views depicted as hostile to the rights of gays and lesbians. Tam testified he likened same-sex marriage to pedophilia, polygamy and legalizing sex with children, and that California would fall into the “hands of Satan” if same-sex couples could wed.

The San Francisco-based Proposition 8 proponent, called as part of the case to overturn the state’s same-sex marriage ban, said that he primarily opposed same-sex marriage because it would encourage children to “marry John or Jane” of the same-sex when they “grow up.”

“I believe if the term marriage can be used beyond one man, one woman, then any two persons of any age, of any relationship, can use that same argument to ask for the term marriage,” Tam testified. “That would lead to incest. That would lead to polygamy. If this is a civil right, what would prevent other groups from asking for the same right?”

With the plaintiffs nearing the end of their witnesses, Tam was called to the witness stand as part of the legal challenge to California’s voter-approved 2008 ban on same-sex marriage unfolding before Chief U.S. District Judge Vaughn Walker.

Plaintiffs’ lawyers consider Tam’s harsh outlook toward same-sex marriage important to proving that Proposition 8 was fueled by hostility and animus toward gays and lesbians, providing reason to give them greater protection under the U.S. Constitution.

Proposition 8 supporters insist they oppose same-sex marriage to preserve the traditional definition of heterosexual marriage, not to discriminate against the rights of gays and lesbians.

Proposition 8 lawyer Nicole Moss, through her questions, tried to distance the campaign from Tam’s statements, depicting him as going rogue with his statements about gays and lesbians without the consent of Proposition 8 campaign officials.

“I was acting independently,” Tam said of statements he made that were not approved by ProtectMarriage.com and Proposition 8’s leadership.

Boies, however, portrayed Tam as inextricably linked to the campaign, showing e-mails in which he regularly communicated with ProtectMarriage.com officials and evidence Tam mobilized the Asian community and organized rallies against same-sex marriage.

The plaintiffs are expected to finish their case today with the testimony of another expert, University of California-Davis psychology professor Greg Herek, who will discuss the nature of homosexuality and its characteristics.

Earlier Thursday, Stanford University political-science professor Gary Segura finished his testimony, which amounted to the equivalent of a full day on the stand.

He was grilled by Proposition 8 lawyers on his position that gays and lesbians are “powerless” in the political arena.

Contact Howard Mintz at 408-286-0236.