What is the deciding factor from one being gay or having a sex change?

Disclaimer: If you can’t approach the question civilly please pass it

For instance if you have Brian who is attracted to men, what makes him want to have a sex change and be Brianna and date men, as oppose to Oscar who is attracted to men but he wants to remain a man attracted to other men. What makes one person attracted to the same sex stay the same sex, while others who are attracted to the same sex desire to be the opposite sex, if you can follow that? I want you to focus on the cause, or plausibility not the validity of the sex act, attraction, or orientation, just what is the nature or cause. Also, be civil about it, and if you have clinical studies please add a link.