Brian Brown, president of the National Organization for Marriage - the leading group opposing same-sex marriage - said those favoring so-called traditional marriage had been outspent by a margin of at least 4 to 1.
"Our opponents and some in the media will attempt to portray the election results as a changing point in how Americans view gay marriage, but that is not the case," Brown said in a statement. "Americans remain strongly in favor of marriage as the union of one man and one woman. The election results reflect the political and funding advantages our opponents enjoyed in these very liberal states."
In Massachusetts, Iowa and Connecticut, laws followed court rulings that same-sex couples could not be denied marriage rights. Legislatures approved the change in Vermont, New York and New Hampshire.
Before this year, ballot initiatives banning the legal recognition of same-sex marriage had succeeded in 31 states, and no state had ever approved same-sex marriage by popular vote.
Maine voters rejected gay marriage in a referendum in 2009 by 53 to 47 percent. In Washington and Maryland, where state legislatures previously passed laws expanding marriage rights to gay and lesbian couples, it was up to citizens to decide whether to let the laws stand.