If they do then that would make them bisexual not a lesbian. If they are a lesbian they are attracted the same sex and only the same sex. Not both guys and girls. Being attracted to guys too would make them bi.
IMO, human sexuality is flexible, so yes, I think that it is possible. There's all this debate as to whether or not humans are biologically wired to be heterosexual, homosexual, or bi-sexual, but I think that humans are just sexual in general.