Lawyers defending California's Proposition 8 and the federal Defense of Marriage Act want the high court to decide it is reasonable for the law to recognize only marriages between opposite-sex couples.

Conservative attorneys did not argue that gays or lesbians engaged in "immoral" behavior or lifestyles. Instead they emphasized what they called the "very real threat" to society posed by opposite-sex couples when they are not bound by the strictures of marriage.

The traditional marriage laws "reflect a unique social difficulty with opposite-sex couples that is not present with same-sex couples — namely, the undeniable and distinct tendency of opposite-sex relationships to produce unplanned and unintended pregnancies," wrote Clement, a solicitor general under President George W. Bush. "Unintended children produced by opposite-sex relationships and raised out-of-wedlock would pose a burden on society."

"It is plainly reasonable for California to maintain a unique institution [referring to marriage] to address the unique challenges posed by the unique procreative potential of sexual relationships between men and women," argued Washington attorney Charles J. Cooper, representing the defenders of Proposition 8. Same-sex couples need not be included in the definition of marriage, he said, because they "don't present a threat of irresponsible procreation."