Why is the name Charlie instantly thought to be a boy

I am 8 weeks pregnant with my second child. This is going to be the last one. I have told a few friends that little Charlie is due in November and the first question is always how do you know it is a boy? My husband is Charles III and they all go by Chuck. He wants our boy to be the IV which is fine by me as long as we call him Charlie. If we have another girl I still want Charlie so she will be Charlotte.