"I don't support the gay marriage," Salmon told KTVK, Phoenix.
"I'm just not there, as far as believing in my heart that we should change 2,000 years of social policy in favor of a redefinition of the family," he said. "I'm not there."
But Salmon, a 55-year-old Mormon and staunch social conservative, said his limit of marriage to opposite-sex couples had nothing to do with his love and respect for his son.
"My son is by far one of the most important people in my life. I love him more than I can say," Salmon said in the interview, excerpted Friday and broadcast in full Sunday.
"It doesn't mean that I don't have respect, it doesn't mean that I don't sympathize with some of the issues. It just means I haven't evolved to that stage," he said.
Salmon voted for the Defense of Marriage Act, restricting federal marriage benefits and requiring interstate marriage recognition to only opposite-sex marriages, and for a ban on gay adoptions in Washington, D.C.
DOMA's constitutionality was argued before the U.S. Supreme Court last week after eight federal courts found part of the law unconstitutional.
Salmon told KTVK he did not consider his son's homosexuality a choice.
"I don't believe that this is a lifestyle that he chose," Salmon said of his youngest son, Matt R. Salmon. "In fact, I remember him telling me at one point in time, he said, 'Dad, do you truly believe that if I could have chosen a lifestyle I would have chosen this, with all the things that come along with it?'"
In a 2010 interview with the Phoenix New Times, the younger Salmon -- 22 at the time and president of the Arizona Log Cabin Republicans, a pro-gay-rights GOP group -- revealed he was dating Kent Flake, the second cousin of Sen. Jeff Flake, R-Ariz.
The younger Salmon has since left the Log Cabin Republicans leadership to concentrate on medical school, and his relationship with Kent Flake has since broken up, The Washington Post said.
Jeff Flake, widely seen as a rising GOP star, told NBC's "Meet the Press" Sunday he believed "marriage should be between a man and a woman" but also predicted a Republican U.S. presidential candidate would back same-sex marriage sooner or later -- and get party support.
"I think that's inevitable," said Flake, a Mormon.
He said he would support such a candidate.
Mitt Romney, also a Mormon and the party's 2012 presidential nominee, said he opposed same-sex marriage but said states could extend some rights to same-sex partners.
The only sitting GOP senator fully supporting same-sex marriage is Rob Portman of Ohio, who told reporters from Ohio newspapers March 14 he switched his stand after his son told him he is gay.
"It allowed me to think of this issue from a new perspective, and that's of a dad who loves his son a lot and wants him to have the same opportunities that his brother and sister would have -- to have a relationship like Jane and I have had for over 26 years," Portman told reporters in an interview in his Washington office.
Sen. Lisa Murkowski, R-Alaska, said last week her view on same-sex marriage was "evolving."
"I've got two young sons who, when I ask them and their friends how they feel about gay marriage, kinda give me one of those looks like, 'Gosh, mom, why are you even asking that question?'" Murkowski, a Roman Catholic, was quoted by the Alaska Star as saying Wednesday in response to a question after addressing the local chamber of commerce.