As David, the conniving stud on ABC's One Life to Live, Tuc Watkins is causing a sensation rarely seen on afternoon TV. David's been sleeping with Tina, who until recently thought (wrongly) he was her brother. ''Not your typical couple,'' the actor says. Watkins, 28, also starred in a steamy infomercial for Gravity Edge, a fitness machine: He's seen breathing heavily in the skimpiest of exercise wear, flexing opposite a curvy babe. ''Oh, you caught the undercurrent of sex?'' he jokes. Success hasn't affected his modest ways. ''I'm still eating macaroni and cheese,'' he says. Nor has fame prompted a name change. Born Charles Curtis Watkins III, he couldn't pronounce Curt as a child, and his first attempt, Tuc, stuck. ''It's cute now,'' Watkins says, laughing. ''I'm not so sure it will be so cute when I'm 45.''