Marlene Dietrich is one of my all time favorite actresses. In her film Morocco, she fought for a scene where she donns a tuxedo, sings a song and kisses a woman in the audience. She single-handedly saved the kiss from the censors by taking a flower from the woman and handing it to Gary Cooper. If they cut the kiss, the whole scene would have had to been scrapped.
For 1930, this was pretty ground breaking. I'm thinking that it may be the first girl kiss in mainstream cinema: