THE NATIONAL PORTRAIT GALLERY’S “Hide/Seek: Difference and Desire in American Portraiture” is an exhibition whose time has finally come. Or so I wrote several weeks ago, before events in Washingtonjust days before we go to presscame to lend my words a bitterly ironic cast. Gays and lesbians have become acceptable, as characters and as actors, on television shows (Queer as Folk, Will & Grace, The L Word) and in major films (Philadelphia, The Crying Game, Brokeback Mountain), as well as in popular music (Elton John, Ricky Martin, Lady Gaga), and the Internet has made every facet of