IMO, TG and sexuality are distinct but related issues.
As a male, I was firmly heterosexual. This really complicated my coming to terms with my being trans, basically for the same reason that many people have trouble understanding that a transwoman can be lesbian. Shouldn't I just be content that I was permitted by society to be attracted to women? Why should I complicate that and risk ostracism? It took me an additional 30ish years to realize that there's a lot more to happiness and gender than who I'm attracted to, and that I can't safely just ignore the persistent need for a female body. I think this'd have been a lot easier had I been attracted to men, though that would have added its own complications.
So, I think it's complicated, and it all fits together somehow, and this is why it's important to make a big effort to understand yourself and to take transition at a slow enough pace that you can self-reflect throughout. It's very easy to misunderstand oneself, but you have to do something. By going slowly, you have time to correct missteps and find your way to where you belong.