When entering a new contact, you can designate each contact as male or female. This allows you to use the keyword search by gender option in the 'Find Addresses' portion of the Address Book.
Please note that if you have gender specific groups in your Address Book (i.e.: Male Clients, Female Clients) but do not designate the gender in the gender section when adding a contact, the keyword search by gender option will not work. Additionally, if you do assign a gender to contacts as you add them in the Address Book, but do not select the gender appropriate group, the contact will not automatically be added to the gender specific group.