Typically, sex isn’t included on a family chart, but there are a few occasions when it is useful. For example, where a child died very young, and wasn’t named, or if someone had a given name that people are more familiar with being of the opposite sex. For example, Philip and Nicholas are traditional mostly male names, but both have been used as female names in the past.
This can be set on the “Main” tab of the “Current Person Other Detail” dialog box, accessed y clicking on the ⋮ button next to the name box.
This flag also includes the sex when listing children in reports.