Mitt Romney, the man likely to be the Republican candidate in this year's US presidential election, has rejected the legitimacy of same-sex marriage.
"Marriage is a relationship between one man and one woman," he told graduates at Liberty University, a Christian college in Lynchburg, Virginia.
Mr Romney, a Mormon by religion, was given a standing ovation.
Barack Obama, fighting for re-election as president in November, announced his support for gay marriage this week.
It was seen as a politically risky move, especially in the South, where one in three swing voters strongly opposes allowing gays and lesbians to wed.
Virginia is regarded as a key battleground in November.
Addressing the graduation ceremony at Liberty University, Mr Romney avoided talking about his own faith but stressed the importance of Christian values in American society.
He has so far struggled to gain support from evangelical Christians in his campaign for the Republican ticket, the BBC's Jonathan Blake reports from Washington.
"There is no greater force for good in the nation than Christian conscience in action," he told the audience.
While Mr Romney opposes gay marriage, he has said that same-sex couples should have some rights, including the ability to adopt children.