The poll was conducted a week ahead of historic US Supreme Court hearings on whether the Constitution allows state restrictions on recognizing the legality of same-sex marriage. The country's highest court is expected to debate the issue April 28.
The court will review cases from four states — Michigan, Ohio, Kentucky and Tennessee — where injunctions on same-sex marriage were overturned by federal courts of appeals.
In March, the Obama administration filed a brief with the Supreme Court stating the White House opinion that it is unconstitutional to ban same-sex marriage.
In 2013, the Supreme Court ruled that the Defense of Marriage Act (DOMA) signed in 1996 by former US President Bill Clinton which prevented the federal government from recognizing same-sex marriages legalized by states was unconstitutional.