It sounds like the lecturer is trying to use the incident as a learning opportunity, aka teachable moment. He is asking you to argue your case in order to get you to think about the implications of your request so that you will learn a valuable lesson unrelated to the specific topic of the assignment. Probably he is reasoning that the current low-stakes situation is an excellent time to learn this lesson because the consequences of making a mistake are not large; by being a bit harsh or annoying now, he can teach you something that is likely to save you much more severe consequences at some later time in your life.
To clarify, I'm not expressing an opinion about whether making such a demand of you is a good idea, but I suspect that this is the reasoning motivating the lecturer's request.
As for how to reply, I suggest just following the lecturer's instructions and coming up with the best arguments you can. I could offer more specific advice, but I think that would defeat the purpose of what the lecturer is hoping you'd do, which is to think about this issue yourself. (Nor do I think there is a universally correct answer, for that matter.) And again to clarify, by saying this I don't mean to imply that there's anything wrong with you coming here to ask the question, but I sincerely think you're better off coming up with your own answers. Good luck!