What makes some females become tomboys?

A tomboy as I understand it does not make a girl a lesbian, as it were, it just means she wasn't as hung up on sex roles at an early age. I think it is a fact that girls have an easier time of it if they are attracted to the playthings of boys, whereas boys who do the same thing, will be persecuted. In general I think rigid sex stereotyping at an early age is just wrong and stupid. Psychological tests have established for a long time that the more intelligent you are, the less hung up you'll be on conforming to some sex stereotype.