Unpopular opinion: the rule is stupid but you ARE a cheater and a liar like he said, his feelings are valid, you’ll be ok too, move on.
Everyone is focusing on the rule being stupid, it is stupid, but you’re still totally wrong in this situation. The rule is stupid because sooner or later you’ll end up in a situation where it’s just not feasible to inform your partner, and this kind of zero tolerance on something like that would never be healthy in a polyam relationship. That all being said you went to see him right before, he plans right after, and still chose to try to be sneaky about it. If I was him I’d think you’re the kind of person that gets off on the cheating aspect, and I would be instantly done too.
Either way you’re incompatible, don’t beat yourself up over it but don’t try to change his mind either, move on.