Redefining marriage to include same-sex couples could adversely affect families and society at large, the lawyer defending California's gay marriage ban told Supreme Court justices today.

"It is reasonable to be very concerned that redefining marriage as a genderless institution could well lead over time to harms to that institution and to the interests that society has always used that institution to address," argued Charles Cooper March 26.

The attorney made his remarks Tuesday during oral arguments for Hollingsworth v. Perry, one of two gay marriage cases being heard this term by the U.S. Supreme Court. This case challenges California's Proposition 8, a state measure recognizing marriage as existing solely between a man and a woman.

The other case, to be argued March 27, challenges the Defense of Marriage Act, a federal law defining marriage as the union of one man and one woman. Decisions in both cases are expected in late June.

Lawyers for both sides of the argument began by addressing questions of jurisdiction, and then moved onto the merits of the case.

Cooper affirmed that the "essential thrust" of the defense of Prop 8 is that gay and straight couples are not "similarly situated" with respect to marriage because the government's "principal interest in marriage is in regulating procreation."

Cooper referred to the "unknown consequences" of gay marriage, and suggested they will be "adverse."

Justice Antonin Scalia added that there is "considerable disagreement" among sociologists about the effects of raising children by gay couples, and that "there's no scientific answer" about it at present.

He made this point noting that if marriage is redefined so as to include same-sex couples, adoption by same-sex couples must be permitted as a consequence.

Cooper responded by saying that the proponents of Prop 8 must prove "not only that there clearly will be no harm, but that it's beyond debate that there will be no harm" should gay marriage be allowed.

Justice Stephen Breyer pointed out that there is more to marriage than procreation, and that couples who are straight yet sterile are afforded the right to marry.

Cooper responded that "redefining marriage as a genderless institution will sever its abiding connection to its historic traditional procreative purposes, and it will refocus the purpose of marriage and the definition of marriage away from the raising of children and to the emotional needs and desires of adults, of adult couples."

He said that marriage is designed to "make it less likely" that marriage partners "will engage in irresponsible procreative conduct" outside of marriage so that should the marriage produce children, they will be more likely to be raised by both parents.

Attorney Ted Olson argued against Proposition 8, saying that it "walls-off" and stigmatizes gays and lesbians "based on their status" and labels "their most cherished relationships as second-rate, different, unequal, and not okay."

Olson proposed treating homosexual persons as a class whose equal protection right to marriage is being infringed upon.

"This Court again and again and again has said the right to get married, the right to have the relationship of marriage is a personal right. It's a part of the right of privacy, association, liberty, and the pursuit of happiness."

Chief Justice John Roberts, however, expressed discomfort with saying that Prop 8 excludes "a particular group," because marriage as a heterosexual institution developed – not out of malice to homosexuals as a group – but "to serve purposes that, by their nature, didn't include homosexual couples."

Olson replied that in adopting Proposition 8, California chose to "exclude gays and lesbians," but Roberts countered that the California Supreme Court's decision striking down Prop 8 was a novel change to an institution "that's been around since time immemorial."

Scalia then chimed in, asking Olson at what point in time he argues that it became unconstitutional to exclude same-sex couples from marriage. Olson suggested that it became unconstitutional "when we as a culture determined that sexual orientation is a characteristic of individuals that they cannot control."

When Scalia asked for a particular time this happened, Olson said it has been "an evolutionary cycle."

"Well, how am I supposed to know how to decide a case, then if you can't give me a date when the Constitution changes," Scalia asked.

The court also heard arguments from the U.S. Solicitor General Donald Verrilli, who claimed that Prop 8 violates equal protection.

The measure, he said, denies the children of gay couples the "stabilizing effect" of marriage and that such children are harmed because, without their parents being married, they "don't have parents like everybody else's parents."

In his rebuttal, Cooper said the case is ultimately about "whether or not it can be said that for every legitimate purpose of marriage, are opposite-sex couples and same-sex couples indistinguishable."

This, he said, "is not a hard question."

Cooper concluded that if "the natural procreative capacity of opposite-sex couples continues to pose vitally important benefits and risks to society," then "that's why marriage itself is the institution that society has always used to regulate those heterosexual, procreative relationships."

The court may end up side-stepping a decision in the case, either by finding that those pursuing the case have no standing to do so, or they may dismiss the case as improvidently granted. In either scenario, the Ninth Circuit Court of Appeals' February 2012 decision striking down Prop 8 would be sustained.