(Through May 4) After the lofty inventiveness of The Lion King and the genital gymnastics of Puppetry of the Penis, you may think you've experienced every innovation puppet theater has to offer. But until you've seen cuddly Kate and pretty boy Princeton doing the nasty, you ain't seen nothing. Welcome to the deliciously demented world of Avenue Q, where Gary Coleman rents rooms, monsters mingle with ''Orientals,'' and, as just one of the insanely catchy tunes goes, ''Everyone's a Little Bit Racist.'' The nonhuman inhabitants include questionably gay Rod and Nicky (read: Bert and Ernie), the Mae West-like Lucy the Slut, and the prickly Mrs. Thistletwat -- all the fantastic Jim Henson-like creations of Rick Lyon, operated on stage by real-life doppelgangers. As they sing, dance, and impart Sesame Street-style lessons on such improbable subjects as porn and homosexuality, you realize you'll never look at puppets the same way again. Especially now that you've seen them naked.