postgresql function that inserts and validates

I'm trying to make a function that inserts a new person from a table and validates the gender of the person (must be 'male' or 'female'). I know how to make a function that inserts a new person and I know how to make a function that raises an exception if the gender is now male or female. But I don't know how to do this in one function and without using a trigger.

What I have already is something like this:

CREATE OR REPLACE FUNCTION InsertPerson(IN _id INTEGER, IN _name VARCHAR(50), IN _gender CHAR(1))
RETURNS void
AS 
$$

  BEGIN

    IF NEW.gender <> 'Female' AND NEW.gender <> 'Male' THEN
       RAISE EXCEPTION 'Gender must be either Female or Male';
    END IF;


    INSERT INTO Person (ID, name, gender)
    VALUES(_id, _name, _gender);


  END;
$$ LANGUAGE plpgsql;

Can someone please help me