Sean Penn is slamming the backlash against straight actors playing homosexuals as “timid and artless”. The ‘Mystic River’ actor and director, 63, earned critical acclaim and his second Oscar for best actor thanks to his portrayal of the openly gay politician Harvey Milk in 2008’s ‘Milk’, and has now said he has been “miserable” on movie sets ever since. He told The New York Times about how he could not star in the film today amid hot debate over heterosexuals playing queer characters: “No. It could not happen in a time like this. “It’s a time of tremendous overreach. It’s a timid and artless p…