I'm not even wanting to go that far yet. My point is, it is very hard to create a single, concise definition of what makes a boy and what makes a girl.
If you go by "sex organs", then what about a hermaphrodite? Do you say if the organs resemble male more than female (51% male) then the person is a boy? Yes, there are few "true hermaphrodites" where it is that close a call, but they do exist. Additionally, if you solely use this definition, doesn't that mean, with surgery, someone should be able to change gender.
Of course, then you get "it isn't that simple" argument -- and they start to go to internal sexual organs. And, granted, this can be helpful when determining what to do with a child that doesn't have clearly defined outer sex characteristics. For example, I know a girl who was born with no vagina, though she does have ovaries and a (misshapen) uterus, so she has a surgically created vagina today (just like a transsexual) -- which if for no other reason was needed for menstruation. But, if you use this definition, does that mean that people who have hysterectomies or other types of (accidental or intentional) castration no longer have a sex? Or, again, what about children born without working sex organs (no working ovary or testicles) -- or in cases where both seem to exist (either functional or non-functional). Also, does the brain count as a sexual organ (since there are noted differences between brains of the average male and female)?
And, of course, most will claim that "science" would claim sex is what DNA states that it is. The issue here, we then have (from birth) people who have XY chromosomes and female sex organs, or people with XX chromosomes that have male sex organs. And that doesn't get into X0 (only one sex chromosome), XY, XXY, XYY, XXYY, etc.
My point is that "male" and "female" is not as easy as most people try to claim it is, particularly if you are going to claim "science."