Sean Penn shares an onscreen smooch with a male co-star in his latest movie role, playing one of America’s first openly gay politicians.

Penn’s co-star Mark Martinez claims the Oscar winner embraced the kiss, and threw himself wholeheartedly into the liplock.

Penn plays San Francisco City Supervisor Harvey Milk in the Gus Van Sant directed Milk.

Martinez, who appears as cross-dressing disco singer Sylvester James, tells PageSix.com, "I’m performing, and (Penn) comes onto the dance floor.

"He grabs me, and he just slaps the biggest kiss on me…It felt like the kiss was forever. I’m like, is he going to stop? I had to close my eyes, I couldn’t believe it."

He was so taken with the kiss, he asked Van Sant to reshoot the scene: "I’m thinking, we gotta do this thing again. We just didn’t get it right. But Gus was like, ‘It’s perfect, perfect, perfect.’ I said, ‘No! It’s not perfect! Sean was laughing at me!’"