Why is there a connection between Judy Garland and the gay population?

Judy Garland was one of the first major stars to acknowledge gay fans. From her character in the Wizard of Oz, the phrase "friend of Dorthy" emerged as a self-identifier of gay men in the mid-20th century. So I guy might ask another in the bar, "Are you a friend of Dorthy?" if he said yes that meant he was also gay.