As someone who refuses to believe that Bisexuality actually exists (having dated a handful of supposed Bisexuals), I find it even harder to accept Pansexuality.
I believe that there is an open-mindness to experiment with the same or opposite gender, but that there is a clear and obvious bias towards one gender. That gender is closer to the persons latent sexuality, be it gay or heterosexual.
For example, one of the Bisexuals I became intimate with claimed that she liked men and women equally. But tbh, the sex did not feel natural. It felt as if she was confused and never quite conformed to the stereotypical female role. Despite her confession that I was able to satisfy her, she had me convinced that she preferred women. I'm surprised I made it as far as the bedroom with someone like her.
I have more examples to opposite scenarios for obvious reasons. Their arguments seem to be a variation of 'girls are fun to play around with, but after a while, you miss what it feels like to be with a man". It seems to me that girls of this type would hook up with other girls just as a sexual outlet for the time being while they look for what they really desire, a man. Once they are with a man, they don't care anymore about hooking up with girls... unless they wouldn't mind sharing their guy with someone else. I wonder if there would be as many 'bisexual women' as there are now if it weren't almost socially acceptable.
The bottom line is, I've yet to meet someone who is truly bisexual at least when it comes to girls. Men are a whole other story. Even the homosexual community is of the general belief that a bisexual man is simply a gay man that is too afraid to come out of the closet. Then again, I haven't met a single bisexual man in my life. At least in person.
Good luck trying to convince me that Pansexuality is more than just the occasional experimentation with transgendered people. I'd still lump that under homosexuality, as I believe those who seek out these types are more interested in the persons original gender rather than their new one. Otherwise, they'd look for naturally born men or women.