I like the idea of this, but I also completely agree with Skeezyks. This all said, the ability to share something like this is already present. Just share it on your profile as part of your biography-type thing. And you can add the (M) to your tag that shows on your profile and posts. You could go so far as to request a name change that would suggest your gender. Lastly, there is the signature which is shown (if enabled for the user) within posts you make; you could add a gender thing there.