Nobody is arguing that a male and a female are generally necessary to have a child. People are just saying "I was born a male and I'm not comfortable with the role of the 'man' in society, so socially I'd like to be treated as a 'woman'". That's it. They aren't demanding that their doctor pretend they have female chromosomes. They aren't demanding that you call their penis a vagina.
This is only confusing to people who want to maintain the confusion as as shield for bigotry, at least past a certain point. I was certainly genuinely confused at first, but it only took one serious explanation for it to be really easy to understand. There is no excuse if it has been explained 500 times and you stay confused.