At least one Stepford wife really has a secret: He's a man! Readers of gay magazines like Out have noticed a dapper gent in ads for ''The Stepford Wives.'' Paramount's mum, but he's ''The Producers''' Roger Bart, who plays half of a same-sex couple in ''Stepford.'' Screenwriter Paul Rudnick says he aimed for ''a new twist on the original.'' Which begs the question: Will this help or hinder their quest for a marriage license?