This is a great request. It would be best to start off with a short definition of relativism. Relativism states that there are no absolute truths. "Truths" are relative based on perception, social conditioning, and other very human factors. Hence, what is acceptable in one culture may not be acceptable in another culture.
In light of this, we can argue for the legalization of marijuana on many levels. At the most basic level we can say that marijuana is not universally wrong, because there are no absolutes.
Perhaps a more cogent line of reasoning would be to compare marijuana with alcohol and tobacco. We all know that alcohol impairs judgment and is addictive, but it is still legal. We can say the same with tobacco. In light of this, we can make the argument that marijuana, at least, has medical benefits! So, why ban one substance and not another?