Hollywood actor Sean Penn has played many great roles, including that of gay rights icon Harvey Milk for which he won an Oscar in 2009.

Since then, Penn acknowledges there has been much progress made with LGBT rights and marriage equality.

But in a recent interview with HuffPost Live, he played with a straight bat, saying he was surprised at how “slowly” attitudes are changing in American culture despite the legal and political gains.

He warned anti-gay parents about the impact their attitudes will have on future generations.

We all have to work hard and be patient with how fast you can move with certain things relative to the backlash it’s going to take,” he said.

And the longer it takes for attitudes to shift, he warns the more destruction of young lives homophobia will cause.

He fears things will change “not before children in this generation are going to have children and they’re going to look back at (their parents) and say, ‘My parents are responsible for the suicides of 13 and 14 year old boys and girls.’

The sooner we change it, the better our kids are going to feel about us.”

Penn, 54, said he was in his 20s when he became acutely aware of the damaging impact homophobia has on the lives of gays and lesbians.

You have friendships with people who are experiencing oppression and the prejudice of it or the violence of it – the isolation.

Your brain starts to realise, ‘What is up with this?’ Your first thing is to say, ‘This has to change yesterday.’

So I’m glad it’s changing,” Penn said.