The Republican National Committee proclaimed that marriage was a "relationship that only a man and a woman can form" and urged the Supreme Court to rule against same-sex marriage in two key cases before it.
The RNC "affirms its support for marriage as the union of one man and one woman, and as the optimum environment in which to raise healthy children for the future of America," said a resolution passed at its Spring Meeting.
It rejected the "unfounded accusation that support for marriage is based only on irrational prejudice against homosexuals," while imploring the top US court to "uphold the sanctity of marriage" in landmark rulings.
President Barack Obama last year came out in favor of same-sex marriage, which is the subject of California's Proposition 8 and the federal Defense of Marriage Act, both being considered by the Supreme Court.
Following Obama's November election victory, the Republican Party has been grappling with how to reach out to liberal voters on issues like same-sex marriage, abortion and immigration, without alienating its conservative base.
The choice of famously liberal Hollywood for its spring meeting was symbolic -- as acknowledged by RNC chairman Reince Priebus in his keynote address before the end-of-meeting resolutions.
"Welcome to Hollywood! That's not something you hear RNC chairmen say very often," said Priebus. "It really is great to be here in Hollywood. Or as President Obama would call it, Real America."
The long list of Obama's A-list re-election supporters included George Clooney, Robert De Niro, Samuel L Jackson, Scarlett Johansson, Leonardo DiCaprio, Morgan Freeman and Robert Redford.
Republican Mitt Romney's celebrity backers were far fewer in number -- the most high-profile, Clint Eastwood, raised eyebrows by talking to an imaginary Obama in an empty chair at the Republican convention in August.
Former vice president Dick Cheney spoke at this week's Hollywood meeting, but his appearance was not publicized, in what may reflect a desire to downplay the party's hardline conservative wing.
But party officials were keen to announce the appointment of a national Asian and Pacific Islander field director and communications director, as Republicans seek to improve their outreach to minorities.
"This is about going where we haven't been, listening to voters we haven't heard, competing in regions where we haven't in a long time. You know -- like Hollywood," said Priebus.
He also invoked the spirit of late British premier Margaret Thatcher, who was famously close to Ronald Reagan, a Republican president who hailed from traditionally Democratic California. She died on Monday at the age of 87.
"While we have to do things differently, there's one thing that can't and won't change: our principles," Priebus said.
"To paraphrase the great Margaret Thatcher, whom the world has honored this week, 'The chairman's not for turning.' I'm for changing minds -- not changing values."