A few points:
1 - The gays love to claim famous people as their own.
2 - There are many gays in the entertainment business - not only as performers, but as management, agents, writers, company bosses etc. For the young and upcoming there have always been incentives to be accommodating, if not actually on the team.
3 - Many people aren't really hetero or homo, or even bi - they're "not-fussy-sexuals." This is not generally admitted because so many people a big deal about belonging to their sexual orientation.
4 - In the past (and still today) there were enormous differences between certain celebs' public image and their real lives. Even in the 80s an awful lot of people were astonished when it became known that Rock Hudson was gay. When someone you've never imagined was gay gets outed, stories about other celebs become that much more credible - or seem to, anyway.