Blankenhorn was presented to the court as an expert witness in Perry v. Schwarzenegger by the defendant-intervenors, the proponents of California Proposition 8 , a constitutional amendment stripping same-sex couples of the right to marry. The plaintiffs objected to Blankenhorn as an expert witness, but Judge Vaughn R. Walker reserved the question of his qualification and permitted him to testify.

On cross-examination by David Boies, Blankenhorn stated that marriage's "rule of two people" is not violated by polygamy, because "Even in instances of a man engaging in polygamous marriage, each marriage is separate. He ... one man marries one woman." Several times, the judge instructed Blankenhorn to answer the questions he was asked, saying "Let's have a question and an answer", "There is a question pending, Mr. Blankenhorn", and " ... because I'm sure you would not want your demeanor on the stand to be a negative factor in your testimony, I would urge you to pay close attention to Mr. Boies's questions and to answer them directly, succinctly".

Blankenhorn's book Future of Marriage presents views which were echoed in his testimony at the trial. During questioning, he stated "I believe that adopting same-sex marriage would be likely to improve the well-being of gay and lesbian households and their children."

In his decision filed on August 4, 2010, Walker ruled that Blankenhorn was not qualified as an expert witness and that his testimony was unreliable and should be given "essentially no weight," noting among other things that "Blankenhorn's concern that same-sex marriage poses a threat to the institution of marriage is . . . undermined by his testimony that same-sex marriage and opposite-sex marriage operate almost identically".

The Judge concluded that "None of Blankenhorn's opinions is reliable," writing that "Blankenhorn gave absolutely no explanation why manifestations of the deinstitutionalization of marriage would be exacerbated (and not, for example, ameliorated) by the presence of marriage for same-sex couples," and that "Blankenhorn's opinions are not supported by reliable evidence or methodology and Blankenhorn failed to consider evidence contrary to his view in presenting his testimony."