Sean Penn wins best actor Oscar for "Milk"

LOS ANGELES (Reuters) - Sean Penn took home the Oscar for best actor on Sunday for his portrayal of slain San Francisco gay rights activist Harvey Milk in the movie "Milk" and used his win to defend the right of same-sex couples to wed.

Penn, 48, picked up the second Oscar of his career, the first being his 2004 win for his lead role as a grieving father in "Mystic River."

"You commie, homo-loving, sons of guns," the sharp-tongued Penn told the audience as he collected his award. "I did not expect this and...I know how hard I make it to appreciate me."

The straight actor's portrayal of an openly gay politician was a timely one -- with "Milk" coming out shortly after California's same-sex couples lost their right to marry in a voter referendum.

"I think it is a good time for those who voted for the ban against gay marriage to sit and reflect and anticipate their great shame and the shame in their grandchildren's eyes if they continue that way," Penn said.

"We've got to have equal rights for everyone."

Penn, who has a tough guy image dating back to his early career scrapes with the paparazzi, convincingly evoked the legendary charm of Milk, who was shot to death in 1978 at San Francisco City Hall.

"He came in kind of ready made" for the role, openly gay "Milk" director Gus Van Sant said.

In presenting Penn as a nominee, storied actor Robert de Niro joked: "How, for so many years, did Sean Penn get all those straight roles?"

Several critics called the performance the best of Penn's already storied and versatile career, which took off after his breakthrough surfer-stoner role as Spicoli in "Fast Times at Ridgemont High."

This year's best actor race was thought to be a battle between Penn and Mickey Rourke, an actor who scored a remarkable career comeback as an aging athlete trying to remain in the ring in "The Wrestler."

"Mickey Rourke rises again," Penn said. "And he is my brother."

Penn won the Screen Actors Guild award for best actor earlier this year and a slew of critics' prizes. The Oscar nomination for "Milk" was his fifth in the best actor category.

(Reporting by Mary Milliken; Editing by Cynthia Osterman)