Randall Munroe on Google+’s mandatory selecting and display of your gender when creating your profile:
Many women grow up with a sense of physical vulnerability that’s hard for men to appreciate. Our culture’s relentless treatment of women as objects teaches them that they are defined by the one thing that men around them want from them—men who are usually bigger, stronger, and (like any human) occasionally crazy. This feeling—often confirmed by actual experiences of harassment and assault—can lead, understandably, to a lifetime of low-level wariness and sense of vulnerability that men have trouble appreciating. A male designer building an interface should try to keep in mind that there are reasons a female user might feel uncomfortable being told she has to broadcast her gender.