class String
  def sex_code
    if ["male", "男"].include? self
      0
    elsif self == "女"
      1
    else
      self
    end
  end
end

p String.instance_methods.grep(/^sex/) # => [:sex_code]

p "male".sex_code  # => 0
p "女".sex_code  # => 1
p "other".sex_code # => "other"