Firstly, if the sole intent of the programmer is to translate:
0->'male',
1->'female'
otherwise -> 'unknown'

…then his refactoring #2 would be my preference.

However, I have a hard time believing that the business rules of the system are not using that gender code for making policy decisions. My fear is that the if/else/switch chain that the author was asking about is replicated in many more places within the code. Some of those if/else/switch statements might switch on the integer, and others might switch on the string. It’s not inconceivable that you’d find a if/else/switch that used an integer in one case and a string in the next!

The proliferation of if/else/switch statements is a common problem in software systems. The fact that they are replicated in many places is problematic because when such statements are inevitably changed, it is easy to miss some. This leads to fragile systems.

abstract class Gender 
{
    abstract string ToString();
}

class Male : Gender 
{
    override string ToString() => "male";
}

class GenderFactory 
{
   Gender CreateGenderByName(string genderName) 
     => genderName switch {
         "male" => new Male();
         "female" => new Female();
         "unknown" => new UnknownGender()
     };
}

interface State { ... }

interface Plugin {
    State init();
    State processValidInput(byte[] input, State state);
    State processInvalidInput(byte[] input, Exception error, State state);
    void finish(State state);
}

no4>class GenderFactory 
no4>{
no4>   Gender CreateGenderByName(string genderName) 
no4>     => genderName switch {
no4>         "male" => new Male();
no4>         "female" => new Female();
no4>         "unknown" => new UnknownGender()
no4>     };
no4>}
no4>

module GenderTypes =
    type Gender =
        | Male
        | Female

module GenderUtils =
    open GenderTypes

    let genderResult x =
        match x with
        | 1 -> Ok Male
        | 2 -> Ok Female
        | _ ->
            sprintf "'%d' - неопеределенное значение пола" x
            |> Error

    let genderChoice x =
        match x with
        | 1 -> Some Male
        | 2 -> Some Female
        | _ -> None

    let unwrapGender (g: Gender option) =
        match g with
        | Some x -> x
        | None -> failwith "пол неопределён"

open GenderTypes
open GenderUtils

let m = genderChoice 10
let g = unwrapGender m

(defgeneric q (a))

(defmethod q ((a string))                  
     (print a))

(defmethod q ((a integer))                  
     (print a))

(defmethod q ((a (eql 0)))                  
     (print "Method called with eql a 0"))

(defmethod q ((a (<; 0)))                   
     (print "Method called with < 0 a"))