What is a pandaka?

I can understand that within the social context of the historical Buddha, being homosexual or bisexual could have possibly been damaging to the Sangha's image as there is still a lot of gay/bisexual/transgender discrimination today. On Wikipedia, it said a pandaka is unable to attain enlightenment. It sounded like a pandaka might be someone who swings both ways to fulfill sexual desires but I am unclear. What about someone who has sexual relations with men but who is heterosexual? Possibly with the shedding of an identity view we lose our interpretation of ourselves as one way, but I don't know. I probably need to learn Pali.