So, do you think that people who buy weed while under the age of 21 should face legal repercussions? And likewise for people who sell weed without checking for ID?
If so, then you also believe in legal penalties for marijuana use; you're just disagreeing about who those penalties should affect and what form they should take.
I don't believe that people who buy weed under the age of 21 should face any legal repercussions.
I believe that people who sell weed without checking for ID should face licensing repercussions. This is how regulation of almost every other industry works. Chicken farmers, beauticians, and mortgage brokers obey the rules (usually) not because they are afraid of being sent to jail, but because they are afraid of losing permission to practice their business.
If so, then you also believe in legal penalties for marijuana use; you're just disagreeing about who those penalties should affect and what form they should take.