bi is a shortened name for
bisexual,
bisexual is a sexual orientation when a person feels sexual desires with both genders.
BUT, if a
bisexual women (for example)had a relationship with a female they are not a lesbian, they are bisexual, if they have a relationship with a person of the opposite sex, they are still bisexual non the less.
bisexuals are not ‘unfaithful’, bisexuals are not confused.