@Meetmeinlove I was in two minds about this as I totally get your vegetarianism, but then I also think that children, when they are old enough to decide, have a right to make their own choices and as its their home too, as long as they do not rub it in your face, they should be able to eat meat discreetly on occasion and we always hope that they learn to cook in their family home. So I can see both sides here.
Nonetheless, your house rule is long‑standing, clear, and based on your values, you’re not stopping them eating meat, just asking that it isn’t cooked in your kitchen. That’s not unreasonable but its their home too.
At 14 and 18, they’re not toddlers. They live there, they eat there, they use the kitchen daily, and they’re developing independence. Wanting to cook their own food, including meat, is normal teenage behaviour and they are not guests, they are residents in their home.
I have thought about this from both perspectives and came out with this.
This boundary isn’t random, it’s identity‑level for you, its not, "I don't like the smell "or "I would rather you didn't". Its a lifelong ethical stance, a disgust response, a core part of your identity and a consistent rule you have presumably set from their birth? That said, teenagers often act first and think later and all test the boundaries at some point. I doubt it was malicious, more convenience and thoughtlessness. I’d frame it as: “You know the rule, you know why it matters to me, and you need to respect it. If you want to cook meat, that’s fine but not in this kitchen and always tidy up after yourselves.”
At 14 and 18 they’re old enough to respect rules, and definitely old enough to clean up properly. Using your chopping board and leaving raw meat remnants around is grim regardless of anyone’s dietary choices. For me, and the reason I have come down on your side, is because of the disrespect, both for the mess they left and the assault on your ethics and not the meat itself.
You’re not unreasonable to be upset, but a firm reset of the boundary should sort it for now, although I suspect its probably not the first time they have done it and won't be the last.