Backers of a gay marriage ban in California struck back at witnesses for the plaintiffs during day three of the first federal case to challenge the constitutionality of a gay marriage ban.

The trial began on Monday in San Francisco with the emotional testimony of a gay and a lesbian couple who have been denied the right to marry in California because of Proposition 8, the gay marriage ban approved by voters in 2008. Jeffrey Zarrillo said he loved Paul Katami “probably more than I love myself.” And Sandra Stiers testified that she was a plaintiff in the case because she wanted to marry her partner of ten years, Chris Perry.

On Wednesday, Letitia A. Peplau, a social psychologist at the University of California, Los Angeles, took the stand. Peplau's UCLA bio says her “research focuses on topics at the intersection of gender, sexual orientation, and intimate relationships.”

Peplau testified that her research showed that “on average the level of quality is the same” as measured by closeness, love and stability between gay and straight couples.

She also challenged the notion that legalizing gay marriage would have a detrimental effect on heterosexual marriages.

“I have a hard time believing that a straight couple is going to say, 'Gertrude we've been together for 30 years, but now we have to throw in the towel because Adam and Stuart down the street are getting married,'” she said.

But during cross-examination, Proposition 8 lawyers managed to get Peplau to concede to the possibility that gay men are less monogamous than heterosexual men and lesbians.

Citing a 25-year-old paper in which Peplau wrote “sexual exclusivity may be more the exception than the rule in gay male relationships,” Nicole Moss asked, “You write about a study that says 36 percent of gay men said it's important to be monogamous, versus 70 percent of lesbians, 80 percent of straight women and 75 percent of straight men. Is it still a fact that less gay men believe that monogamy is important?”

Peplau agreed, saying “as a generalization that the percentage differs,” but then added that the paper was outdated, and written at a time when “gay relationships were much more secretive.”

In an attempt to shore up the defense's argument that marriage is designed primarily for procreation, Moss questioned Peplau about the children of such marriages.

The questioned flummoxed Peplau who eventually responded, “If your question is if two lesbians can accidentally, spontaneously impregnate each other: not to my knowledge.”

Chief U.S. District Judge Vaughn R. Walker also viewed a video deposition of San Francisco resident Hak-Shing William Tam, a defendant in the case, who discussed a letter he wrote to Chinese-American church groups in support of Proposition 8.

Tam wrote that legalizing gay marriage is part of a broader gay agenda that includes the legalization of sex with minors.

“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.

Opponents of Proposition 8 say the measure – approved by 52% of voters – was approved out of animus towards gay men and lesbians. Backers disagree, saying they only wanted to preserve traditional marriage.

The trial resumes Thursday.