hi eljay,
my psychologist was trained in south africa - even received a scholarship at rhodes university (is that right?) to finish his phd there - and he now specialises in GLBT issues in australia, and believes that the one person can be attracted to both (or many) genders/sexes.
if your T was adamant about this issue - then i would think it's a personal prejudice. i know when i was in queer circles a lot of ppl would be uncomfortable with me being bi instead of one or the other, because it brought up issues for them. doesn't change my sexuality, however. and someone denying that it's even possible won't change it either .