Bisexuals have always been a part of as well as apart from the lesbian and gay communities. It is important that bisexuals are included to accurately describe the larger gay community. People leave relationships for all kinds of reasons, not just the gender of their partner. Anyone unable to make a commitment to a relationship may use a person of any gender to leave it. Bisexuals are not heterosexual.
Many people are unaware that identifying as bisexual is even an option.
Many bisexually behaving people do not identify as bisexual for a number of reasons including fear of discrimination and social stigmatization from both heterosexual society and the lesbian and gay communities.