I'm really glad you've found someone you love
But I think you have to accept she feels differently about drugs than you do.
I'm in a weird position because I've been on both sides of this one. I was the sober one for years while all my friends and my ex dabbled in all sorts (and much worse than just a toke). And it drove me mental worrying about people, feeling what they were doing was wrong, that they needed help, because I didn't understand the mentality whatsoever.
Few years down the line and the tables turned. I dabbled myself. And now I have a better understanding of the mentality, even though I don't do the level of things they do, what they do doesn't bother me anymore. I get it. And I know that it's completely out of my hands.
You have to accept the things you cannot change.
And the truth is, if someone has tried a drug like weed and had fun on it and is not 'addicted' to it, or it is not causing their life any problems... They are not going to have much of a reason to say no to it at a party.
At the moment, the only problem it's causing her life is that it upsets you. And while that may be a good enough reason for her to wann TRY and stop for you, because she doesn't understand what actually bothers you about it (because once you've crossed that line your whole thinking changes), it's probably hard for her to justify saying no and giving up having a good time just to satisfy your dislike of it.
So IMO, you have two options:
1. You try to explain to her properly why it bothers you this much and why it's so important that she gives it up for you
2. You accept the fact your girl has the occasional joint.