Interesting point there are not two genders, I once visited Amnesty International HQ and they told me there are third genders, intersex. People that have parts from both genders. And everybody knows about those ladies in Thailand. We have to accept there are more than two genders.
Intersex is indeed very real and can be a very difficult thing for those born that way to live with. Often times many operations and counseling is required for an intersex person to find some semblance of acceptance and peace with their own body and identity. It is not however a gender in its own right as it works on a scale ie. one intersex person might be 5% female/95% male while another could be 60% female/40% male etc etc... and a lot of this is down to interpretation by medical experts and/or the intersex person themselves.
Biologically speaking there are two genders, male and female, intersex falls into a small grey area of about 1-3% of people (depending on what information you read) that are born with a combination of both genders in an undetermined ratio.
The most important thing about this? Intersex has nothing at all to do with the trans movement. Two completely different things. Intersex is a biological phenomenon. Trans is about how somebody feels and while there is absolutely room in our society for courtesy and consideration for people's feelings, it can't be at the expense of scientific fact.