I spent a while trying to find a simple answer to this. I think it’s most easily interpreted as:
Bi: Implies you like both of the genders. No real preference.
Pan: implies you recognize there is a range of masculinity and femininity, and of course cis and trans, and thus you are attracted to a range of genders. Not explicitly feminine or masculine, but likes anyone on the spectrum of genders.
Oooh I had not seen that. With this in mind, I have to just assume that pan came about not knowing that bi had already by definition not limited to the two typical genders.
I had not thought bi to be transphobic, nor that bisexuals actually fit into interest in just two separate genders. I just thought it was perhaps an outdated term that sounds that way. Thank you for the clarification!
There really needs to be more discussion on this stuff. I only recently discovered the terms gynosexual and androsexual. Those could have been super useful when I was younger.