here is a good site to study about a lot of things.
It's especially useful now that Chelsea Manning has decided she no longer wants to be known as Bradley.
http://www.t-vox.org/index.php?title=Trans_101
The thing about sexuality is, it is different from gender. a man may start out as a straight man, fancying women. he may then undergo hormone replacement therapy which will change his/her body chemistry and brain chemistry and his/her soft tissue body structure, and she ends up fancying men.
In effect the straight man, became a straight woman. on the other hand, he may start out fancying women and become a lesbian.
There are lots of possibilities.
unfortunately, what can be taught in schools is extremely limited and usually kept over simplistic. I was also taught that the world consisted of xx and xy.