Here's a bit of circular logic for you: if there are only two genders (hence bi-sexual), and a pansexual is attracted to people regardless of gender, aren't pansexual people just bi-sexual? Pansexuality only exists if one already subscribes to the idea that there are more than two genders. The argument is pointless because it is superfluous to the argument over how many genders there really are: we need to figure that out first.
I feel like the meme's point stands even in light of the existence of pansexuals. I think where the meme is misguided is talking as if sexuality is attraction to a certain gender identity, rather than to sexual characteristics, which do exist in a kind of binary.
I'm a trans man myself, I don't identify as anything other than a human male. I used to think there were more than two genders, but I think different now. While there are two genders, there are people who don't feel strictly male or female.
No liberals want to change the world by any means necessary for a little glory at the end of the day. They're volatile and cause trouble for everyone else.