Because the most vocal and visible pop stars are women who sing about love and cute guys and other "girly" things, or in some cases about loving and accepting yourself "no matter gay, straight, or bi", so it follows in the minds of the public that males who like these stars and their music must relate to the music and like the things the music is about.
Because most pop music is sung by women. Since it's a woman singing it and the lyrics are about a man or just being sexy, when a man likes a song or just hears it, the word gay pops up for many.
