What differentiates a guy from a girl? A mustache, well grown beard , and certain bodily features are the main differentiations between a guy and a girl. The sex of a child is decided even before it comes out of the mothers womb.However, for it to be fully developed, the course depends on some hormones and secretions. Deficient testosterone can be a reason for this.Now that we have natural testosterone replacements that would work the same way in the body’s the original secretions would do, there is nothing to worry about them anymore.