Well it's wrong because it's upsetting to you. He needs to respect your feelings. Does he have a reason for getting other guys' numbers? Is he trading notes from class? Because that's about the only legitimate reason to do something like that. You need to bring it up and let him know that you understand that to him getting another guys' number isn't a big deal, but to you it's undermining your relationship together and you're having a hard time feeling secure in your relationship with him because of it. He either needs to explain clearly what his intentions are by getting numbers, find some way to reassure you that he's not looking for greener pastures, or just stop getting other guys' numbers.
Good luck!
