SQL Server 2012 :: Conditional Logic Function To Return VARCHAR Value With Gender

I'm trying to convert the query immediately below into a function with the conditional logic to return a VARCHAR value with the gender: male, female or unknown.

SELECT empid, firstname, lastname, titleofcourtesy,
CASE
WHEN titleofcourtesy IN('Ms.', 'Mrs.') THEN 'Female'
WHEN titleofcourtesy = 'Mr.' THEN 'Male'
ELSE 'Unknown'
END AS gender
FROM HR.Employees;
GO

Below is the conditional logic function I'm trying to create to replicate the logic above.

CREATE FUNCTION dbo.Gender
(
@male AS VARCHAR(10),
@female AS VARCHAR(10),
@unknown AS VARCHAR(10)
)
RETURNS VARCHAR(10)

[Code] .....