There is a perceived correlation between tomboys and lesbianism. While some tomboys later reveal a lesbian identity in their adolescent or adult years, masculine behavior typical of boys but displayed by girls is not a true indicator of one's sexual orientation. "Throughout their history, tomboys have had to contend with the stigma of presumed lesbianism or the accusation of wanting to be male. Both assumptions were categorically refuted by twentieth-century psychology, which established the normality of the tomboy experience among girls of all identities. However, for many, the tomboy stage is the first manifestation of a gender-fluid life journey."

Historically, tomboys have been defined, as suggested in the examples mentioned above, by "boyish" behavior (like more physically active, technological, and scientific interests) and wearing boys' clothing. In recent times, as the ubiquity of traditionally female clothing such as dresses, blouses and skirts has declined among Western females, the distinction has become more and more one of behavior. A general increase in the popularity of women's sporting events (see Title IX) and other activities that were traditionally male-dominated, is today broadening tolerance and lessening the impact of "tomboy" as a pejorative.

Just to clarify what is meant by 'traditionally masculine clothing', a girl wearing jeans and a t-shirt is not a tomboy, she's normal. A tomboy would be wearing any non-girly top, a baseball cap, trainers, and slacks and/or boys jeans. Tomboys tend to be too busy doing stuff to listen to music.

Info from Wikipedia, last paragraph by me. Image from Photobucket.