The argument against slip that his logic would include smoking weed with rape, murder... ect... doesnt work because crims arnt necessaritly unethical. Pot smoking is toatally ethical and an unethical crime, like Jim Crow acts.
But anyways, if your dad has it that obvious, fair game id say. Only because its your dad. Does anyones family here not share all the food? you cant just say 'This Oarnge Juice is Mine' because you all use a bit of it. If your dad makes it dificult for you to come forward about it, thats his fault. Id say take very small amounts, not too frequently. Theres rules to this smokey game we play, and one of them is 'never leave your shit out for everyone and not expect it to get stolen' . Your dad im sure has played the game for a long time, so he should know how it goes. I woudlnt call it being a theif, more like 'circumstaintial skimming' . Plus, its safer this way as you wont be dealing with dealers.