Security assist will not add any validation to the registration form, so it must be the way your users table is created. In the database, double check the users table to see if the Gender column is set to not allow nulls.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.