With the US Supreme Court due to rule on the hot button issue in a matter of days, the Washington-based Pew Research Center surveyed 1,504 adults on the topic during the first five days of May.
Seventy-two per cent agreed when asked if legal recognition of same-sex marriage was "inevitable" -- including 59 per cent of those who said they opposed allowing gays to marry legally.
Eighty-seven per cent of respondents personally knew someone who is gay or lesbian -- and predictably, support for the legalisation of same-sex marriage was strongest within that group.
Nineteen per cent said they would be "very upset" if their child revealed being gay or lesbian.
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Pew conducted its survey shortly before Rhode Island, Delaware and Minnesota became the latest of the 12 US states plus the District of Columbia to legalize same-sex marriage.
The constitutions in 31 of the 50 US states, as well as the federal Defense of Marriage Act (DOMA), still define marriage strictly as a union between a man and a woman.
Pew posted its findings on its website: www.Pewcenter.Org.