Changes in sexuality (orientation) do happen for many during transition, but certainly not all. For me my orientation only changed labels. I have always, and continue to be, attracted to women. Before, when I was labeled a man, I was considered heterosexual. Now that I'm a woman, I'm labeled a lesbian. I'm perfectly happy and proud to take that title, along with transsexual.
It seems to me that more often younger transitioners tend towards being heterosexual, while us older folks tend to be lesbian/gay. Of course I know many who defy the rules. Fortunately each and every one of us is a unique and complex individual.
There are so many things to look forward to in the years ahead. Sit back and enjoy the ride!