There are a number of articles on Google Scholar that discuss gender and iconography. Skimming the abstracts, it would appear that scientists have tested icons for gender, and each gender has more success with icons that reflect their identity.
Even though the icon you show is an abstraction, it could also be read as "default male" in societies where baldness is in the domain of men, but not women. (Not all societies have this rule.)
Thus, it's a practice to not use a "user" icon at all when gender is unknown, and use initials as an avatar. Everyone has a name.