Kagan cut in: "I can just assure you, if both the woman and the man are over the age of 55, there are not a lot of children coming out of that marriage."
At another point, Justice Antonin Scalia, who has dissented in the court's previous gay rights cases, invoked the well-being of children to bolster Cooper's case.
"If you redefine marriage to include same-sex couples, you must permit adoption by same-sex couples, and there's considerable disagreement among sociologists as to what the consequences of raising a child in a single-sex family, whether that is harmful to the child or not," Scalia said.
The California case was argued 10 years to the day after the court took up a challenge to Texas' anti-sodomy statute. That case ended with a forceful ruling prohibiting states from criminalizing sexual relations between consenting adults.
Kennedy was the author of the decision in Lawrence v. Texas in 2003, and he is being closely watched for how he might vote on the California ban. He cautioned in the Lawrence case that it had nothing to do with gay marriage, but dissenting Justice Scalia predicted the decision would lead to the invalidation of state laws against same-sex marriage.
Kennedy's decision is widely cited in the briefs in support of same-sex unions.
The California couples, Kris Perry and Sandy Stier of Berkeley and Paul Katami and Jeff Zarrillo of Burbank, filed their federal lawsuit in May 2009 to overturn the same-sex marriage ban that voters approved the previous November. The ballot measure halted same-sex unions in California, which began in June 2008 after a ruling from the California Supreme Court.
Roughly 18,000 couples were wed in the nearly five months that same-sex marriage was legal and those marriages remain valid in California.
The case is Hollingsworth v. Perry, 12-144.
Associated Press writer Jessica Gresko contributed to this report.
Follow Mark Sherman on Twitter at: www.twitter.com/shermancourt
Follow Jessica Gresko on Twitter at: www.twitter.com/jessicagresko
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