That's because the AirVPN team didn't write a forums software from scratch, they picked an existing software and adapted it to the special needs of their infrastructure. A gender field in users' profiles is not a special need, given that 98% of people around here don't bother changing profile settings, let alone edit their profile.
IP.Board is a "generic" forums software which can be used in many environments. In some of them contact info, birthdays and genders make sense. In some of them, including airvpn.org, they don't.