I think a key consideration you need to make is whether you trust that on nights out where you aren't there, that she will stick to not taking it.
Personally I think that it's impossible to police others behaviour within a healthy relationship.
If someone stops doing something because the other doesn't like it then one would hope it would be a case of "shit I didn't realise that made you feel bad - obviously I don't want to do anything that makes you upset and that's more important to me than the thing in question". That's a sensible and fair conversation.
But if ends up being a drawn out negotiation like it has done with you two and one side concedes at the last minute to avoid breaking up, it feels forced and unhealthy.
She'll be annoyed she's had to stop doing something she likes so will become resentful and you'll now be paranoid she'll slip up in your eyes because you know she's only agreed it to hang on to you.
If she does it again, she'll say sorry and that she won't but you'll trust her even less and will have given the impression that while you say it's a dealbreaker it's not. But it is something you'll continue to butt heads on.
I think having to make a rule about something like drugs because you are on such different pages about it naturally is a sign of basic incompatibility.
And I'm not being judgemental about either side I promise. As I say I'm an ex addict and have been on both sides of this fence.