麻烦nil值和if语句

Question

我在使用nil值和if语句时遇到问题。当变量sexualPreference为零时，程序会打印出“Sexual preference is nil”和“nil”。这显然是错误的。我想知道为什么会这样？非常感谢任何帮助！

databaseRef.child("users").child(user.uid).observeSingleEventOfType(.Value, withBlock: { (snapshot) in
    let mainStoryboard: UIStoryboard = UIStoryboard(name:"Main", bundle:nil)

    if let sexualPreference = snapshot.value!["interestedInWomen"] {
        print("Sexual Preference is not nil")
        print(sexualPreference) 

    } else {
        print("Sexual Preference is nil")

    }

    // ...
}) { (error) in
        print(error.localizedDescription)
}