I always enjoy this topic when it gets discussed, and a lot of the comments seem to see changes in sexuality as acknowledging attraction that was previously repressed. Relieved repression may be an accurate way of describing what happens to those who, like me, seem suddenly to find ourselves attracted to men after months on hrt. But I also wonder whether this explanation denies potential connections between prolonged administration of cross-sex hormones and attraction. We as transsexuals are essentially going through a new puberty. Admittedly I haven't read up on the subject, but I imagine that many cisgender girls have no measurable sexual attraction to boys before puberty. Then, the hormones are raised for a prolonged period of time and sexuality becomes, well, something. Solidified? Awakened? galvanized? I'm hesitant to name it. And this sexuality that gets realized (I like that word) at puberty need not be hetero, of course. We can then perhaps see why giving testosterone to a gay man won't make him a straight one. He never was a straight man and why giving him estrogen over a long period of time might or might not make him a lesbian.
Again, maybe I was in denial about my attractions to men, but I just can't get access to it in all my memories of youth and adulthood.