If they are a transexual man then wouldn't they be a woman? Legally and well.. as far as their gender is concerned they are a woman.
Pansexual means little more than attraction to any male or female, even if their sex now is different than the sex they were born with. I think it's a fine term actually. For instance I really can't imagine myself with FtM (though holy shit I have seen some cute/hot FtM who have thrown me for a loop) just saying...