There are certain things that are not sinful in themselves but seemed sinful because the society reasonably outlaws them. For instance a 16 year-old having one beer is not a sin but becomes a venial sin by going against the drinking age. Or this comes up often regarding the morality of Marajuana.
I think the teaching is fairly clear in this regard but I have trouble finding a clear source or a better argument. Now a friend is challenging me on this. Can you please help me out?