I hate coke, but if he's not buying it, and has simply snorted it at a party when you weren't around it wouldn't be a deal breaker for me.
You were unreasonable for saying 'never do it again' you should have said 'never pay for it or do it anywhere near me' as that is something you're reasonable to expect.
I used to love hash, but decided at a certain age that I didn't want it anymore (becoming a parent), that I wouldn't buy it anymore, and that I wouldn't do it as part and parcel of a night in/out anymore. I've been 'out' a few times when a joint's been passed round and off my own back, I've declined. I've also been out and had a toke or two and enjoyed it immensely, a one off, no damage done. Had that been something that would have destroyed my marriage, I'd have to not only question my failed 'promise' but the reasonable 'control' of my partner.
Some people CAN dabble, without becoming a nasty piece of work. You don't have to LIKE the fact he snorted some coke, but he didn't buy it, bring it home, and its not a habit, nor is it an out of contril habit.
I wouldn't like it, but to chuck away a whole relationship because of one good night out seems a little off.