Supreme Court Justice Anthony Kennedy joined conservative colleagues in asking skeptical questions Tuesday as the high court heard historic arguments over the right of gay and lesbian couples to marry.

Kennedy, whose vote is seen as pivotal, said marriage has been understood as one man and one woman for "millennia-plus time." He said same-sex marriage has been debated in earnest for only about 10 years, and he wondered aloud whether scholars and the public need more time.

"It's very difficult for the court to say 'We know better,'" Kennedy told Mary Bonauto, a lawyer representing same-sex couples.

Chief Justice John Roberts said gay couples seeking to marry are not seeking to join the institution of marriage.

"You're seeking to change what the institution is," Roberts said.

The arguments offered the first public indication of where the justices stand in the dispute over whether states can continue defining marriage as the union of a man and a woman, or whether the Constitution gives gay and lesbian couples the right to marry.

The session was interrupted after about 30 minutes by a protester yelling loudly. He was removed by security.