In some ways, the term “pansexual” came out of biphobia and a need to stipulate that one was not transphobic. If you take the binary view of “bisexual,” then a sexuality specific to an attraction to men and women could be seen as being noninclusive of transgender men and women. On the other hand, transgender men and women want to (and should) be seen as simply men and women, meaning that they would/should be included in that very binary; not including them tends to be much more phobic and noninclusive.
Then there is the thought that the binary view of bisexuality can be seen as phobic of anyone who identifies as genderqueer, or somewhere along the gender and sexuality spectrum, not identifying as male or female, man or woman. But, as I mentioned before, the true definition of “bisexual” is being attracted to those who are the same as me and those who are different from me, encompassing all genders and identities. The often-repeated argument that “bi means two” ignores a simple fact: “Same” and “different” are, indeed, two groups.
via The Bad ‘B’ Word: A Need for Bisexual Acceptance
THE OFT-REPEATED ARGUMENT THAT “BI MEANS TWO” IGNORES THE SIMPLE FACT: “SAME” AND “DIFFERENT” ARE, INDEED, TWO GROUPS