I don't see anything wrong with the phrasing. It acknowledges that society places us into culturally defined sex categories. It would be better if it asked for gender too. There is a much higher prevalence of stuttering among males than females, thus there is believed to be a genetic component, thus your assigned sex at birth is relevant. If this was a research study I would definitely want to account for important variables such as transgender status and gender identification, as well as people born with both male and female sex organs. However, I believe that is outside the scope of this little survey, unless it's actually for a research study in which case they would be breaking informed consent ethics by not informing us - which is highly unlikely since IRB's are generally on top of these things.