This is an interesting topic and one that is becoming blurred in the sporting community. I think there should be, as
@jrry32 mentioned an official acceptable levels of testosterone / oestrogen for athletes. I remember an olympic scandal when I was younger where the IAAF did menstruation tests on female athletes from Russia and Germany to 'check' if they were female, could this be something?
Personally I believe if you are born a particular sex, that's what you are. But there are people that throw that theory out of the window. Caster Semenya for instance is 'Intersex' -
she was born with both sets of sexual organs and has a high level of testosterone naturally.
There are a number of international female athletes have publicly said that they have NO chance of beating Caster because of her genetic make-up. I think it's unfair on the born female athletes.
Have you seen Caster compete, she really does look like a man competing in the women's race.