I think women get that way because of two things;

1. MJ makes you lazy and you will never accomplish anything while smoking it
2. it is an outside interest you don't share with her



The first you could prove wrong quite easily by keeping a job or going to school, the second you could take care of by smoking out with her, buying her paraphanilia and taking her shopping at some nice head shops.

The actual key is to ask her why she doesn't want you too, then fuck up her reasons. The only one that is valid is the illegality aspect, that one I understand. If you can prove to her that smoking weed doesn't affect you in those horrible ways she may chill out and smoke it with you.