Attorney General Eric Holder said state attorneys general are not obligated to defend laws in their states banning same sex-marriage if they don't believe in them.
Holder made the remarks Tuesday morning at a winter meeting of the National Association of Attorneys General.
He said decisions not to defend state laws should not be steeped in political disagreements and instead "must be reserved only for exceptional circumstances."
But Holder said "we must be suspicious of legal classifications based solely on sexual orientation" and that upholding equal rights is an important principle.
Democratic attorneys general in five states -- Pennsylvania, Virginia, California, Illinois and Nevada -- have declined to defend same-sex marriage bans against lawsuits filed by gay couples.
Pennsylvania Attorney General Kathleen Kane announced last June her decision not to defend Pennsylvania's law banning same-sex marriage, two weeks after the United States Supreme Court struck down the federal ban.
"Denying equality is the very definition of discrimination," she said.