If you don't wish to disclose your gender to other forum members, you can do so by adjusting your profile privacy preferences. To do this, click the Profile button found in the forum's top menu bar, and then on your profile page click the Edit Profile button. You'll then be taken to an Edit Profile & Settings page, which once there, click on the Privacy tab. When the page loads, the first option you will see will be for Show Gender; simply select the No option if you do not want your gender to be known on the forum. Once this is set, click the Save Privacy Settings button found at the bottom of the page to complete the process.
When you have your gender hidden, this will prevent all visitors of the forum from seeing your gender. This includes forum staff members such as the main admin of the forum.