What began as a single day of pride commemorating the Stonewall Riots of 1969 that sparked the Gay Liberation Movement has grown to a whole month: In 2000, June was designated an official month of celebration and memorials.
Music wasn't as integral to this movement as it was to the Civil Rights movement, but the movement did impact scholarship and creativity in music. Long-dead composers whose private lives remained private have been "outed" and some living musicians have declared their sexual orientation publicly.
"Queer" theory has played a role in recent music scholarship, and sexual identity has become a topic of scholarly inquiry in music:
Queering the pitch: the new gay and lesbian musicology
ML 55 .Q44 1994
Queer Noises: Male and Female Homosexuality in Twentieth Century Music by John Gill
ML 63 .G49 1995
Songs in Black and lavender : race, sexual politics, and women's music, by Eileen Hayes
ML 82 .H42 2010
Several of the Twentieth Century's top composers were gay. You can hear the variety of styles they represented on Compact Disc 4419, Gay American Composers and Compact Disc 4904, Lesbian American Composers