HRT had NO effect on my orientation (I identified as a baby dyke from early on). What I can easily believe happens for some transsexuals is that as they begin their transition, they lose the firm grip on some internalized homophobia that might have existed, whether they realized it was there or not. Let's face it, there are some who engage in traditional opposite-sex relationships simply to fulfill the societal norm and not because they were really attracted to the opposite sex.
I have also known natal people who were bisexual who simply did not have an absolute affinity for one sex or another. Not having had that feeling of fluidity, I cannot say if it really exists or if they are just willing to sleep with whoever will have them in their beds (almost a co-dependency thing).