I was faced with the same problem three and half years ago, and I just came out and told her "Hey I smoke weed and I know you don't approve of it but if you really like me you wont give me any hell cuz this is who I am and this is what I like to do." She cried for about a day and got over it and now we are happily married and she is just as big of a pot head as I am. Now my wife would like to tell her side of the story so I am handing the keyboard over for a few minutes.
First off, he was drunk and high, and we were instant messaging each other on yahoo. I asked him what he had been doing all day and his reply was, " drinking and smoking." I totally flipped out on his ass and he told me that if I didn't like it then to break up with him. Needless to say I didn't and instead I tried it myself and have been smoking with him every since.