Opponents of same-sex marriage in California rolled out their star witness Tuesday, an author and advocate who predicted that allowing gays and lesbians to wed would discourage heterosexual marriage and might lead to legalized polygamy.

Extending marital rights to couples who cannot conceive children would change marriage from "a child-based public institution to an adult-centered private institution" and "weaken the role of marriage generally in society," David Blankenhorn, founder of the Institute for American Values, testified at a trial in San Francisco federal court on the constitutionality of the state's ban on same-sex marriage.

Blankenhorn, the trial's last scheduled witness, said he believes "leading scholars" share his view that same-sex marriage would weaken heterosexuals' respect for the institution and accelerate a half-century-old trend of increased cohabitation and rising divorce rates.

But under cross-examination by a lawyer for two same-sex couples, Blankenhorn was unable to cite any supporting statements or evidence for that conclusion from the scholars he relied on for his testimony, though he said he was sure some of them would agree with him.

Plaintiffs' lawyer David Boies also pointed to a passage in Blankenhorn's 2007 book, "The Future of Marriage," that appeared to contradict his entire position.

"We would be more American on the day we permitted same-sex marriage than we were on the day before," Blankenhorn wrote.

He said Tuesday he still holds that view, and also believes that allowing gays and lesbians to marry would probably be good for the couples and their children.

Blankenhorn offered no explanation for his seemingly divergent views. A lawyer for Proposition 8, the November 2008 ballot measure that outlawed same-sex marriage, may ask Blankenhorn to explain his position when he returns to the stand today.

Prop. 8's defenders called Blankenhorn to answer a question posed by Chief U.S District Judge Vaughn Walker when he ordered the trial: How does a ban on same-sex weddings protect marriage, the stated goal of the initiative?

The plaintiffs presented academic witnesses who said allowing gays and lesbians to marry would benefit the couples and their children and improve the status of marriage without affecting opposite-sex couples.

The ballot measure's defenders said it merely reaffirmed traditional marriage, but their chief witness for Prop. 8's benefits was Blankenhorn, who has spent two decades arguing against same-sex marriage. His 1995 book, "Fatherless America," contended that attacks on the traditional family had weakened the bonds between fathers and children.

Scholars overwhelmingly agree, he testified Tuesday, that marriage is "a socially approved sexual relationship between a man and a woman" and that the "optimal environment" for children is with their married, biological parents.

Allowing gays and lesbians to marry would further the "deinstitutionalization" of marriage, Blankenhorn said, and increase pressure to legalize polygamy.

Walker allowed him to testify as an expert on marriage over the objections of Boies, who pointed out that Blankenhorn has no academic degrees related to the subject. The institute he founded in 1987 describes itself as a nonpartisan organization dedicated to "contributing intellectually to the renewal of marriage and family life."