That may be true. It does seem to be true that one's physiology is largely determined by DNA alone. If you want to define ones sex as determined by one's physiology you'll be able to get by as long as you don't try to apply such a simple definition to real people. If sex = physiology, then there is something else we can call sexuality. What seems to be the case is that one's sex is not always consistent with one's sexuality. And have you considered that both sex and sexuality may be determined by DNA! But does that matter? What would seem to matter most is that one can live their life according to their sexuality, regardless of their physiology.