Show array of meta_value in Edit Post Coloum

The error was here: (Thank you @Rarst)

 if ($column_name == 'custom_checkbox_group_sesso') {
            $sesso_occupanti = get_post_meta($post_ID, "custom_checkbox_group_sesso", true);
          //check that we have a custom field
          if ($sesso_occupanti != "")
          {
            // Separate our comma separated list into an array
            $sesso_occupanti = explode(",", $sesso_occupanti);
            //loop through our new array
            foreach ($sesso_occupanti as $sesso)
            {
              echo $sesso ;
            }
          }
            }; 

All I had to do was to delete this sentence:

$sesso_occupanti = explode(",", $sesso_occupanti);

Because, as @Rarst said:

if you are storing array then you get back array as well and don’t
need to explode it.