Ошибка сериализация бинарного файла

System.Runtime.Serialization.SerializationException: 'Type 'FitnesCenter.BL.Model.Gender' in Assembly 'FitnesCenter.BL, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' is not marked as serializable.'  

Ошибка возникает при вызове метода Save(), подскажите в чем причина? Пользователь создаеться без ошибок, а запись не проходит, хотя файл каким то мусором занят.

using System;
namespace FitnesCenter.BL.Model
{
    [Serializable]
    public class User
    {
        /// <summary>
        /// UserName 
        /// </summary>
        public string Name { get; } // Setter не делаю, для того что бы нельзя было поменять имя пользователя
        public Gender Gender { get; }
        public DateTime BirthDate { get; }
        public double Weight { get; set; }
        public double Height { get; set; }
        public User(string name, 
                    Gender gender, 
                    DateTime birthDate, 
                    double weight, 
                    double height)
        {
            #region checkInputParameters
            if (string.IsNullOrWhiteSpace(name))
            {
                throw new ArgumentNullException("Name gender can't be empty or null.", nameof(name));
            }
            if (gender == null)
            {
                throw new ArgumentNullException("Gender can't be empty.", nameof(gender));
            }
            if (birthDate < DateTime.Parse("01.01.1900") ||  birthDate >= DateTime.Now) //TODO : mayby need its write like this - "DateTime.Parse("1900/01/01")" 
            {
                throw new ArgumentException("Impossible date of birth.", nameof(birthDate));
            }
            if(weight <= 0)
            {
                throw new ArgumentException("Weight can't be less than or equal to zero.", nameof(weight));
            }
            if(height <= 0)
            {
                throw new ArgumentException("Height can't be less than or equal to zero.", nameof(height));
            }
            Name = name;
            Gender = gender;
            BirthDate = birthDate;
            Weight = weight;
            Height = height;
            #endregion
        }
        public override string ToString()
        {
            return Name;
        }
    }
}

using System;
namespace FitnesCenter.BL.Model
{
    /// <summary>
    /// Делаю не Enumom что бы удобнее было работать с EntityFramework
    /// </summary>
    public class Gender
    {
        public string Name { get; }
        public Gender(string name)
        {
            if (string.IsNullOrWhiteSpace(name))
            {
                throw new ArgumentNullException("Name gender can't be empty or null.", nameof(name));
            }
            Name = name;
        }
        public override string ToString()
        {
            return Name;
        }
    }
}

using FitnesCenter.BL.Model;
using System;
using System.IO;
using System.Runtime.Serialization.Formatters.Binary;
using System.Xml.Serialization;
namespace FitnesCenter.BL.Controller
{
    /// <summary>
    /// User Controller
    /// </summary>
    public class UserController
    {
        public User User { get; } // In it I will write
        /// <summary>
        /// Deserialize
        /// </summary>
        /// <returns>User data</returns>
        public UserController()
        {
            var binFormatter = new BinaryFormatter();
            using (var fs = new FileStream("users.dat", FileMode.OpenOrCreate))
            {
                if (binFormatter.Deserialize(fs) is User user)
                {
                    User = user;
                }
            }
            //TODO: Что делать если пользователя не прочитали
        }
        public UserController(string userName, string genderName, DateTime birthDate, double weight, double height)
        {
            #region checkInputParameters
            if (string.IsNullOrWhiteSpace(userName))
            {
                throw new ArgumentNullException("Name gender can't be empty or null.", nameof(userName));
            }
            if (string.IsNullOrWhiteSpace(genderName))
            {
                throw new ArgumentNullException("Gender can't be empty.", nameof(genderName));
            }
            if (birthDate < DateTime.Parse("01.01.1900") || birthDate >= DateTime.Now) //TODO : mayby need its write like this - "DateTime.Parse("1900/01/01")" 
            {
                throw new ArgumentException("Impossible date of birth.", nameof(birthDate));
            }
            if (weight <= 0)
            {
                throw new ArgumentException("Weight can't be less than or equal to zero.", nameof(weight));
            }
            if (height <= 0)
            {
                throw new ArgumentException("Height can't be less than or equal to zero.", nameof(height));
            }
            #endregion
            var gender = new Gender(genderName);
            var user = new User(userName, gender, birthDate, weight, height);
            //New option
            //User = user ?? throw new ArgumentNullException("User can't be null.", nameof(user));
            //Old School
            if (user == null)
            {
                //Возможно, проверка не требуеться, лишняя... // мы все проверили зареанее!
                throw new ArgumentNullException("User can't be null.", nameof(user));
            }
            User = user;
        }
        /// <summary>
        /// Serialization
        /// </summary>
        /// <returns>True if recording success</returns>
        public void Save() // It's necessary to indicate what we serialize 
        {
            //Serialize User
            //Encrypt file
            BinaryFormatter binFormatter = new BinaryFormatter();
            using (var fs = new FileStream("users.dat", FileMode.OpenOrCreate))
            {
                binFormatter.Serialize(fs, User);
            }
        }
    }
}