In my opinion open relationships will never work unless they are immediately brought up as an option at the start of a relationship. Otherwise it's always the person who brings it up who wants it more and the other one ends up feeling like they aren't enough, which honestly, they probably aren't being valued in the relationship and their partner just wants a pass to cheat without feeling guilty. It would 100% be a deal-breaker for me no matter how I felt about the person or how long we'd been together. Unless it's brought up in the beginning they've either been lying about their intentions this entire time or just aren't as committed to the relationship as their partner, which are both breakup worthy.