If you have a service where you publish your name, link people together into social networks, engage in video chat, post pictures, post status updates and converse with people, gender will become obvious very quickly, regardless of whether the male/female box is ticked.
Since you seem to believe that posting your gender online is analagous to walking around in downtown Bagdhad, and that women are incapable of conducting themselves in modern society, what other measures do you recommend we take?
Should my wife have to wear a Nixon mask while video-conferencing? Does she need to censor parts of photographs that would allow people to determine that she is a woman? Should she be not allowed to join groups or follow people of interest to other women?