If you define bisexuality as “an attraction to men and women”, then define pansexuality a “an attraction to men, women, and transgender individuals”, you are using transphobia to support a biphobic idea of bisexuality.
Simple as that.
The transphobia should be obvious; making the distinction between men, women, and transgender individuals invalidates trans men and trans women as ACTUAL men and women. It also places all transgender individuals in this “other” category. Not man, not women, no matter how they identify their gender. Trans women are actual women, not an “other” kind of woman. Trans men are actual men, not an “other” kind of men. Nonbinary people can identify under the terms “men” and “women” if they choose to, and they will still be just as valid as men and women as another individual.
The biphobia may be a little less obvious, however. Bisexuality is not defined as an attraction to cis men and cis women (saying so actually invalidates many bisexual people and aids in the invalidation of actual trans/nonbinary bisexuals), it is defined by the majority of the community as a “attraction to two or more genders or same and different genders”. The act of claiming otherwise (on a community level, not a personal definition level) is erasive and contributes to biphobia within the LGTBQ++ community.
Your argument that “bisexuals are attracted to cis men and cis women” not only invalidates trans men and trans women, and nobinary/trans bisexuals, it results in two very dangerous dynamics in the LGTBQ++ community.
Firstly, it creates tension between the bisexual and transgender/nonbinary communities. Historically, this is a fairly recent tension, as trans activists and bisexual activists often worked hand in hand during the first trans rights movement. This dynamic shreds apart two of some of the closet knit communities, and forces transgender people to identify as labels other than bisexual, else they face a wave of biphobia from their fellow transgender community. Despite this (thankfully), a large amount of transgender individuals identify as bisexual, and have begun doing what they can to end the faulty idea that bisexual is binary.
The second dynamic is constant fighting between the bisexual and pansexual community. Opinions about umbrella term usage aside, these communities have an enormous amount of overlap that requires them to work together to fight ideas of monosexism; a social system that operates on the belief that single gender attraction is the standard, and enforces this by rewarding SGA and punishing MGA. Gay/Lesbian monosexism operates in a different environment than hetero monosexism, but both cause strife towards the MGA communities.
These dynamics are reinforced daily by all members of the LGBTQ++ that continue to allow the bisexual definition of “attraction to cis men and cis women” to spread and be taken as the real definition. Until the rest of the LGBTQ++ gets on board and begins to allow bisexuals to define their community, and their individual forms of attraction, we will not be able to heal the fracturing relationships between BT and P.