What drives sexual development is not separate. As far as having a "sex change" where one attempts to say that they have changed their sex what is inaccurate is to only focus on how the external genitalia appear and say that alone can change sex. Sexual characteristics do include physiological characteristics brought about by sexual development, but even so a transsexual person does not have the anatomical or physiological characteristics of their desired sex. A man that has surgery to change the penis and accompanying region to appear female does not have a vagina. What is created is not a vagina, but merely something that looks like one. The same is true for a FTM transsexual. Those created "organs" are aesthetic imitations. Physiologically their characteristics are still according to their birth sex, their body unaided by treatment will function as it does according to their birth sex and they will need to be on constant hormone therapy.
Let's go through this again:
Does DNA determine sex? Yes, your genetic code contains the instructions for your sexual development. XY is male and XX is female.
Are sexual organs part of what makes someone their sex? Yes, a MTF transsexual has a penis that has been surgically altered to look like a vagina, they do not have a vagina nor do they have female organs (uterus, ovaries, etc). A FTM transsexual still has a vagina that has been surgically altered to look like a penis, they do not have a penis nor do they have male sexual organs (prostate, testicles, etc). In both cases the natural sexual organs have been altered by surgery to appear different, but they are imitations of what is real as far as the opposite sex goes.
