Hi so, honestly, there isnt a difference between bi and pan, bi is attraction to all genders, we have two labels that describe the same orientation.
Basically, the pan label was popularised online in the early 2000s. at the time there was a pervasive idea tht bisexuals were only attracted to cis men and women. This is from the first post on the 2002 Livejournal community, I Am Pansexual:
“… [T]his community is for pansexuals, as described in the user info. Pansexuals love people of all genders, male and female, but unlike bisexuals, pansexuals love transgendered, androgynous and gender fluid people, people who don’t fit into the categories of male or female.”
you can find the source for that quote and read more info on the history of pansexuality here. Those who IDed with the pan label back then used pan to mean attraction to cis men, cis women, and trans people. This definition is transphobic, trans men and women arent different genders to cis men and women and therefore, they dont fall under a separate attraction, nonbinary people arent a new third gender that you need a whole new and different sexuality in order to be attracted to. This is generally what people are referring to when they say the pansexual label has transphobic origins, but there have also been several redefinitions of pan that have also been transphobic.
I reccomend watching this youtube video which addresses the narrative that bi activism has a history of ‘rampant transphobia’.
Since 2002 the label has gone through several changes in definition, but the most common one lately seems to be that pan is attraction to all genders (regardless of gender); while bi is attraction to 2 or more (gender is a factor in attraction).
But if we look back at bi history, we can see that bisexuality has never been limited by gender, bisexuals were never fixated on the bi = 2 definition. its also useful to know that when people first started iding as bisexual (Stephen Donaldson, also known as Donny the Punk, was one of the first people to id as bisexual in around 1966 and the label gained popularity in the 70s), it meant something similar to unisex.
Bisexuals have been describing their bisexuality as attraction regardless of gender since the 70s;
“Being bisexual does not mean they have sexual relations with both sexes but that they are capable of meaningful and intimate involvement with a person regardless of gender”
Janet Bode, The Pressure Cooker, View From Another Closet (1976)
“I am bisexual because I am drawn to particular people regardless of gender. It doesn’t make me wishy-washy, confused, untrustworthy, or more sexually liberated. It makes me a bisexual.”
The Bisexual Community: Are We Visible Yet?, By Lani Ka’ahumanu (1987)
and The 1990 Bisexual Manifesto:
“Bisexuality is a whole, fluid identity. Do not assume that bisexuality is binary or duogamous in nature: that we have “two” sides or that we must be involved simultaneously with both genders to be fulfilled human beings. In fact, don’t assume that there are only two genders.”
evidences that bisexuals were aknowledging nonbinary genders (although they would have used genderqu**r or genderfuck instead of nonbinary as nonbinary was coined as a gender term more recently than the 90s) and including them in their bisexuality a decade before the I Am Pansexual community even existed. This is one of the many reasons why the pan label is also biphobic, because it trys to redefine bisexuality against bisexuals will, so that pan can mean attraction to all genders instead of bisexual.
If you want to read more about how different definitions of pansexual are based in biphobia i reccomend this article.
Here is the current defintion of bisexuality, as used by the American Institute of Bisexuality, and the Journal of Bisexuality. from the bi.org website:
“There are as many ways to be bi as there are bi people. No one word can fully capture the nuances of personality, culture, beliefs, or personal preference; no word should try to. The term bisexuality makes no such pretense— it is a general and inclusive term that encompasses everyone; all sexes and all genders. That does not mean that bi people are attracted to everyone, merely that bi people’s attractions aren’t limited by sex or gender.”
“Please also note that attraction to both same and different means attraction to all. Bisexuality is inherently inclusive of everyone, regardless of sex or gender.“
If you want to read more about bi history then this article; The Bisexual History They Don’t Want You to Know by @mediumkravitz is a good place to start. Kravtiz has also written; Does Liking a Nonbinary Person Make You Bi or Pan? Not Necessarily. which might clear up any confusion on where attraction to nonbinary genders fits into sexuality.
Also lastly, if you would like to read more on bisexuality and bi history, i recently made this masterlist of bi books, which might be helpful.