In my opinion, and of course it’s just my opinion, even though I agree they're not 100% straight, their attraction to trans women kind of feels like an extension of their heterosexuality. I mean, they're still only into feminine traits like the voice, the body, everything which are dominant in trans women or ladyboys. So for me, that alone isn’t enough to call them bi or pan, unless one day they decide to try something with a cis guy. As I said, that’s just how I see it. I know not everyone agrees.