I think that our culture divides people into opposites as far as gender goes, not sex. I also think that gender exists on a continuum, and few people sit at either end - most of us move up and down the middle somewhere - and things like cultural norms, historical eras, social class and age influence how 'typical' we are perceived in terms of those cultural expectations, but anyway, it is society that decides what is 'appropriate' male and female behaviour. It's just that most people don't fully conform to one set or norms or the other.
As I have said before, but will take the time to say again, I think that it would be far better for society to stop seeing some behaviours as 'male' and others 'female', so people can just be, and decide for themselves on a pick and mix. The idea that people can feel that they are in the 'wrong' gender simply reinforces the idea that there is such a thing as 'correct' gender behaviour, which I don't accept.
Sex, on the other hand is immutable, and cannot, therefore, be changed.
Oh, and if we are playing at being extremely rude - just as you couldn't care less about my personal life and can't be bothered to explain your unscientific theories about sex, I'm not really interested in your partner's reaction either. I don't have to run things by a man to find my bearings, particularly the partner of someone on the internet.