What types of behaviors do you associate with smoking marijuana? Giggling on the couch while watching reruns of Family Guy? Binge eating an entire box of pizza in one sitting? There are lots of stereotypes towards typical stoner behavior, most of which are not so flattering.
But like wine which can either be used for religious ritual or for drunken debauchery, marijuana has proven medicinal properties...and also meditational ones. Have you noticed that marijuana smokers tend to be chilled out, relaxed and peaceful people? They also tend to be creative and insightful - just think of a few famous stoners: Willie Nelson, Bill Maher, Bob Marley, Bill Gates (!) and many others.
Marijuana can open up planes of consciousness previously inaccessible for the beginner meditator. The state of being "high" is akin to some low states of meditation. Thoughts may come rushing towards you (the state of paranoia some marijuana users report), but these are helpful - they let you confront thoughts from the subconscious which have been repressed. By processing these, you are able to free yourself from their unconscious influence on your behavior.
Saddhus (Indian sages) have been smoking charas (hashish) in chillums for thousands of years and is considered one of the five sacred plants according to the Vedas. Lord Shiva himself was a stoner! Legend tells that one day after an argument with his family, he wandered to a pasture and fell asleep under a leafy plant. Upon awaking, he sampled the leaves of the plant and loved it so much he proclaimed it has favorite food! (History of Cannabis in India)
You can even combine marijuana and meditation into one practice: smoke or ingest your strain of choice, and sit quietly, watching your thoughts, witnessing your breath. Alternatively, you may feel like dancing, practicing yoga or stretching. Enjoy your body and the sensations that the marijuana invokes; it is incredibly pleasant - and that is true meditation - enjoying the body, enjoying the present moment and celebrating existence.