There are so many different factors that can be used to determine gender, but none can define it alone. Although the vast majority of people are genetically, anatomically, hormonally, mentally and and in every other way like their gender; it's not so simple for everyone. My personal opinion is that there are 2 genders, but not everyone is simply one or the other. If somebody wants to categorise it differently, I respect their opinion.

Most people are completely one gender and not the other. Some people are a bit of both genders, some are completely one and partly the other, some are completely both and some are neither. If someone's not 100% biologically male or female and/or not 100% happy with their biological sex, they should be allowed to choose which gender/genders they identify with; and if they want to change their biology to better match their gender identity, they should be allowed to as they can't help feeling that way.