The usual narrative goes something like "I can handle this, but there are lots of people that can't, and we have to keep it out of their hands", though the people in question vary -- drawing class, demographic, or cognitive lines is common.

Or, alternately, "It was a mistake for me to do it, and I was lucky to get away without punishment, but legalizing would encourage other people to make the same mistake." I seem to recall a few U.S. politicians on both sides of the aisle saying things of this nature.