a mirror is just not possible for my cycling purposes sorry

I am trying to figure out this reasoning. What cycling purposes don't allow you to put a mirror on your bike? I believe its illegal to not have a rear view mirror on a car, but if I told a cop it didn't fit into my driving purposes I don't think that would be good enough to not get a ticket.

If your reasoning for riding against traffic is because you want to see what is coming behind you then save up a few bucks and buy a mirror and figure out a way to put it on your bike. The end.