I may be wrong, but I was once explained otherwise.
According to what I have heard:
-Bisexual: you like males who identify as males and females who identify as females. "Bi" means literally "two" and no more.
-Pansexuals: you may like a very wide range of genres, like transexuals (their genitalia is somewhere in the middle between being man or woman), males who identify as females, fluid genre... there are quite a few.
-Demisexual: is not based on the genre but in a different thing, like any other gray sexuality, but demisexuality is the only gray sexuality with a specific name. Demisexuality is based on the emotional relationship with the other person. Other gray sexualities may vary a lot, but I guess they might be about aspect (like hair color or body build), demeanor (there is a film about a woman who couldn't get excited unless the other person was crying, I don't know how based on reality is that), and other things.
As a demisexual, most of us are pretty much not affected to any physical characteristics. They are just that - hair is just hair, a body is just a body until we have some emotion or meaning attached to those characteristics, then we are sexually attracted. With gray ace, it's basically everything between sexual and asexual. Some people are more asexual than sexual (sex disgusts them but they still get aroused by just... hormones etc) but others can be more sexual (they can be aroused by porn and stuff and even have a type). It's not really about specific aspects but maybe for those who lean more towards sexual in the spectrum.
When people dont believe bisexuality exists you get this ....it kind of annoys me to be honest....oh yeah hes totally straight having sex with a man