Controversial witness Hak-Shing William Tam finally took the stand Thursday during the federal trial to decide the constitutionality of Proposition 8.

The trial, now in its second week, opened last Monday in a San Francisco courtroom with the emotional testimony of a gay and a lesbian couple, each of which has been denied the right to marry in California because of Proposition 8, the gay marriage ban approved by voters in 2008.

Lawyers for the plaintiffs have spent the better part of the past two weeks arguing that proponents of Proposition 8 approved the measure out of animus towards gay men and lesbians. The defense says limiting marriage to heterosexual unions fosters a stable environment to raise children.

Tam is the controversial gay marriage opponent who alleged that gay marriage advocates also supported the legalization of sex with children. He has appeared in court on several occasions, but had not been called upon.

During a pretrial hearing, Tam attempted to remove himself as a defendant in the case after learning that Walker had green lit a plan to broadcast the proceedings to other courtrooms. Proponents of Proposition 8 appealed the ruling to the Supreme Court, which blocked the broadcasts.

A video deposition of the San Francisco resident was viewed by Chief U.S. District Judge Vaughn R. Walker last week. In the video, Tam spoke about a letter he wrote to Chinese-American church groups in support of Proposition 8.

“On their agenda list is: legalize having sex with children,” Tam wrote. The letter goes on to warn that “other states would fall into Satan's hands” if the gay marriage ban is not approved.

On the witness stand, Tam said he did not represent the Proposition 8 campaign in an official capacity and called an email naming him a leader “flattery.”

“I don't believe I am [in the leadership],” Tam said.

But plaintiffs lawyer David Boies disagreed. Boies asked Tam whether he devoted a majority of his time to getting the measure on the ballot. Tam answered “yes.” Boies asked whether Tam organized Prop. 8 rallies, and again, Tam answered “yes.” Boies asked whether Tam debated the issue on television. Tam answered “yes.”

Boies hammered Tam on his motivations, including the claims that gay people are 12 times more likely to molest children and that being gay is a choice. Tam said he believed the National Association for Research & Therapy of Homosexuality (NARTH), a group that believes sexual orientation can be altered, over the APA, which says being gay is a normal sexual deviation.

Tam had few concrete answers to explain his prejudice against gay men and lesbians, saying he got some of his ideas from the Internet, but disagreed that he was hostile to the gay community.

Shaken by the cross-examining, Tam suggested that gay marriage would simply “mix up kids.”

Nicole Moss, a lawyer for the defense, countered that Tam met with the Proposition 8 campaign mostly during the signature gathering phase. “I was acting independently,” Tam said.

The case resumes Friday, when plaintiffs are expected to wrap up their case and hand over the courtroom to the defense.