The conservative lawyer who defended California’s ban on gay marriage at the Supreme Court is at work on another project: planning his daughter’s upcoming same-sex wedding ceremony.

Charles J. Cooper, a former top official in the Reagan Justice Department and onetime “Republican lawyer of the year,” learned of his daughter’s sexual orientation during the legal battle over California’s Proposition 8, according to journalist Jo Becker’s soon-to-be-released book chronicling the movement to legalize same-sex marriage.

Ashley Lininger became engaged to a woman identified in the book only as Casey just after the Supreme Court accepted the Proposition 8 case in December 2012. Cooper, a noted Supreme Court practitioner, argued the case in March 2013.

The court ruled against Cooper’s clients, saying they did not have legal standing to challenge a federal district judge’s ruling that the ban was unconstitutional. Same-sex marriages then resumed in the nation’s most populous state.

In its limited ruling, the court sidestepped Cooper’s argument that there is no constitutional right to same-sex marriage and that decisions about whether to allow such unions should be left to the states and voters.

Cooper told Becker that he did not think it appropriate to comment on how he would vote on the issue should he have the opportunity.

“What I will say only is that my views evolve on issues of this kind the same way as other people’s do, and how I view this down the road may not be the way I view it now, or how I viewed it 10 years ago,” Cooper is quoted as saying.

Cooper joins a list of prominent Republicans — former vice president Richard B. Cheney and Sen. Rob Portman of Ohio among them — with children whose interests are at odds with party orthodoxy on gay marriage.

Becker wrote that Cooper and his daughter spent hours discussing the case while it was ongoing and disagreed about Cooper’s view that states had reason to enshrine the traditional definition of marriage in their constitutions and withhold the right from same-sex couples.

“I think the most upset I got was being called an ‘experiment’ that people deserved to see the outcome of before accepting,” Becker quoted Lininger as saying. “It just made me feel — alien, I guess.”

Lininger lives in Massachusetts, the first state to legalize same-sex marriage and now one of 17, in addition to the District of Columbia, where the unions are legal. She did not want to be interviewed for this story. Cooper said the same, although he offered a statement:

“My family is typical of families all across America. We love each other; we stand up for each other; and we pray for, and rejoice in, each other’s happiness. My daughter Ashley’s path in life has led her to happiness with a lovely young woman named Casey, and our family and Casey’s family are looking forward to celebrating their marriage in just a few weeks.”

He added: “As Becker reports in her book, I told Ashley that what matters most is that I love her and she loves me.”

In the book, Cooper said he left it up to his daughter — he married her mother, Debbie, when the girl was 7 and always refers to her as his daughter — whether she wanted to go public with her engagement during the litigation.

Although it might have made the point that personal concerns are different from questions of policy, Cooper said he was relieved she decided on privacy.

“I didn’t want, and I didn’t think she wanted, for her and Casey to suddenly become the most famous lesbians in America,” Cooper told Becker. “But can you imagine how riveting it would have been if at the oral argument I disclosed this? I kind of personified what I was arguing.”

Becker is an investigative reporter for the New York Times who won a Pulitzer Prize for national reporting when she worked for The Washington Post. For her book “Forcing the Spring: Inside the Fight for Marriage Equality,” she was given unfettered access to the legal team seeking to overturn Proposition 8 — the odd-couple team of Republican Theodore Olson and Democrat David Boies — as well as the organization American Foundation for Equal Rights, which was formed to bring a case to the Supreme Court to recognize a constitutional right for same-sex couples to marry.

She did not have the same access to the legal teams defending Proposition 8 and the companion case that challenged the federal Defense of Marriage Act. But she wrote that Cooper granted her hours of interviews and allowed her to tell the story of his family.

Cooper granted few media interviews during the years-long litigation and was criticized by some conservatives for not being vigorous enough in his defense of traditional marriage.

He was haunted throughout the proceedings by a comment he made when the Proposition 8 case was at trial before U.S. District Judge Vaughn Walker. Walker asked Cooper how it would harm the state’s interest in encouraging heterosexual couples to marry and raise families if it also allowed same-sex couples to wed. “Your honor, my answer is, I don’t know. I don’t know,” Cooper eventually replied.

Cooper immediately regretted his words and spent the rest of the litigation trying to take them back. What he meant by the comment, he said, was that same-sex marriage was so new and untried — “an experiment” — that the answer was impossible to know.

Becker writes about how, through the years of litigation, the opposing lawyers and plaintiffs were tied together as the case moved through the process.

Cooper and Olson are old friends, usually part of the same elite conservative legal establishment. Cooper succeeded Olson as head of the Office of Legal Counsel in the Reagan Justice Department.

Cooper said in the book that he came to especially admire the lesbian couple who challenged Proposition 8, Kris Perry and Sandy Stier.

They returned the compliment in a statement about Cooper’s defense of Proposition 8 at the same time his daughter was planning to marry a woman.

“Some may find this contrast between public and private jarring, but in our opinion, loving an LGBT child unequivocally is the single most important thing any parent can do.”

The question of whether there is a constitutional right to marriage that states may not withhold from gay couples could return to the Supreme Court in time for its term beginning in October.

But it is not the Proposition 8 case that has been the impetus for an unbroken line of federal district court decisions that such bans are unconstitutional: The judges instead are relying on the court’s decision in the DOMA case, U.S. v. Windsor , that ordered the federal government to recognize same-sex marriages performed where they are legal.