Earlier quoted context omitted.
You are forgetting that the individual might consider the gender to be private information. Some people might want to use a different gender in different contexts. See the ESPN/Grantland suicide issue recently if you care/dare.
An individual that considers gender to be private information , and that uses different genders in different situations is very unlikely to be using a name that can be classified with a high degree of confidence as one gender. A person using the name "Jack" is unlikely to be assumed to be a woman, even if that person selected "female" from a drop down somewhere. If the same person uses Jack/M and Cindy/F in different…
At least in the UK, Jack is a fairly common shortening of Jacqueline. The only way of determining a users gender is asking them directly, and if a person wishes to enter different values into different systems, all the more power to them.