Novelty

I think some men may find Biker Chicks intimidating, i suppose it's the symbol of independence and free spirit that puts the masculine type out of sink with there interpretation of the feminine species, like what rain said, others find it sexy and a turn on, so just remember, those who are negative towards you are dis-empowered by you, it's time to reverse the psychology and not let it bother you,
but instead let it empower you.