Gender is an issue that we are surrounded by constantly. Ideas about gender shape how a baby is treated from the moment of its birth.
Because gender is so omnipresent, it's also something that people take for granted and don't dedicate much time to trying to understand beyond the cultural norms with which they've been raised.
When confronted with transgenderism, people who have never had to deeply think about gender before might find themselves confused. Things that they took for granted may be questioned or challenged. This is one reason why many advocacy groups for transgender people focus on education.