Bisexual and pansexual identities complicate the binary of gay and straight in ways that both communities have sometimes resisted — accused of indecision by one side and disloyalty by the other, which is a reliable sign that the accusers need the binary more than the truth requires it. The experience of attraction that is not confined to one gender is extremely common and extremely poorly served by the categories available to describe it. Pansexuality goes further, explicitly refusing the notion that gender is even the relevant variable in desire.
Each step builds on the last.