How do I save Multiple fields in a meta box?

Question

I would suggest you define a field names property on your class:

class your_class {
    private $_fields = array(
        'firstname',
        'lastname',
        'sex',
    );

    public function prfx_meta_save ( $post_id ) {

    }
}

Now you can fully abstract your save handler:

foreach ( $this->_fields as $field ) {
    if ( ! isset( $_POST[ $field ] ) )
        continue;

    $value = ( string ) $_POST[ $field ];
    $value = wp_unslash( $value ); // WP adds slashes to all request data
    $value = sanitize_text_field( $value ); // Fix invalid UTF-8, strip tags & line breaks

    update_post_meta( $post_id, $field, $value );
}