Why do these people keep forgetting that pansexuals exist ?

If you asked most people who identify as Bisexual about this, I bet you'd get almost no responses on the order of "I am attracted to exactly two genders because there are exactly two genders." It's just another example of the Right (mis)taking an etymology or anagram as a valid and crushing argument.

"Confirmation bias, also known as observational selection, motivated reasoning, or the enumeration of favorable circumstances, is the tendency for people to (consciously or unconsciously) seek out information that conforms to their pre-existing view points, and subsequently ignore information that goes against them, both positive and negative."-Rational Wiki