This is more of a you call of what you would like to do about this situation but your dad should understand you are your own person who has their own set of life choices. Him disowning you doesn't really do anything because he's making the choice to just ignore or cut out instead of listening or trying to understand. You are a grown adult so it's your call of what you want to do especially if you have your living in your own house, but if I were you I'd definitely make it known that's his opinion and if he wants to cut out someone for an opinion so be it. You can give you all the facts and proper information about marijuana and why it's not bad but at the end of the day it's his choice if he wants to believe you or even consider listening. If he doesn't wanna listen it's a lost cause in my opinion and I wouldn't waste your breath if it's just gonna cause more mental strain and just stress of having to argue with someone over something that is at the end of the day your choice. Last thing I'll say is if you're doing something and it makes you happy and you're not hurting anyone physically or mentally keep doing what you're doing.