Sexually ambiguous rocker David Bowie - one of many stars said to have frolicked under the bedcovers with Mick Jagger - says he has fooled around with guys, but ''I was always a closet heterosexual.'' Bowie, who raised eyebrows in the '70s with his fem-drag ''Ziggy Stardust'' look, insists in Rolling Stone, ''I didn't ever feel that I was a real bisexual,'' even though he was ''making all the moves, down to the situation of actually trying it out.''