asexual = no attraction to either gender.
pansexual = gender doesn't factor any more than a person's hair colour, making gender a non factor. Contrast this with bisexual, which still strongly recognises gender, and the person's feelings towards each are distinct.
Precisely speaking, gender shouldn't be relevant to sexual orientation, only sex. There may be additional levels of gender orientation, but heterosexual, bisexual, homosexual and asexual should be sufficient to describe sexual attraction between non-intersex persons:
Heterosexual: attracted to the opposite sex
Bisexual: attracted to both sexes
Homosexual: attracted to the same sex
Asexual: attracted to neither sex
There are, of course, many degrees between these.