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.

Read the rest of Bob Egelko’s San Francisco Chronicle article here.