When President Barack Obama became the first president to endorse same-sex marriage, he was reflecting evolving American attitudes on the issue. As with other historic civil rights gains for racial minorities and women, Americans eventually embrace fairness. This week, the U.S. Supreme Court will hear arguments in two landmark same-sex marriage cases. The justices should declare that the U.S. Constitution's equal protection guarantees extend to the right of gay and lesbian couples to marry. That would be a just conclusion and reflect where the nation is headed.Hollingsworth vs. Perry, to be heard Tuesday, challenges Proposition 8. That referendum passed in California in 2008 and repealed the right of same-sex couples to marry that had been granted by the state's courts. Wednesday's case, United States vs. Windsor, challenges the federal Defense of Marriage Act. That is the 1996 law that bars the federal government from recognizing legal same-sex marriages. The law denies gay and lesbian spouses a range of federal benefits, including Social Security and tax breaks reserved for married couples. It also tells the nine states plus the District of Columbia that have legalized gay marriage that their marriage laws won't be recognized at the federal level.While the high court should not base its judgment on popular sentiment, it cannot be blind when public views rapidly change in favor of civil rights. A decade ago only 36 percent of Americans supported legalizing gay marriage. Now, 58 percent support gay marriage, including 81 percent of adults under age 30, according to the latest Washington Post/ABC News poll. Historically, the high court has been willing to help catalyze social change when society was already moving in that direction. At one time as many as 41 states prohibited interracial marriage. In 1967, when the Supreme Court ruled those laws unconstitutional, it nullified the 16 laws that remained.The best result would be for the court to recognize that the Constitution's guarantees of equal protection grant same-sex couples the same rights of marriage that heterosexual couples enjoy. That would alter the law of marriage for 41 states, including Florida. With an ideologically divided court and Justice Anthony Kennedy as the sole moderate, the high court is likely to stop short of such clarity. There are numerous alternative grounds to set aside of both Proposition 8 and DOMA.The Obama administration suggests in an amicus brief in the Proposition 8 case that once a state has granted same-sex couples all or nearly all the benefits of marriage, the only basis for refusing to allow gay couples to marry is to segregate them and treat them differently due to their sexual orientation. Putting up legal barriers simply to discriminate against gays and lesbians isn't a legitimate basis for lawmaking. If the court accepted this view, only nine states would be impacted.DOMA could be set aside on federalism grounds on the theory that the federal law interferes with the traditional role of states in defining marriage.There is momentum on the side of expanding marriage rights. An array of mainstream interests from major corporations to high-profile Republicans have weighed in on behalf of same-sex couples. In the end, only nine justices' opinions will matter. They should embrace equality and fairness.