Something happened, and you are unsure whether you are allowed to feel betrayed — because the relationship was never defined. That in-between is one of the loneliest places to be, and there is a way to think it through.
Short answer: whether it counts as cheating depends on what was understood between you, and if that was left unsaid, you and they may have been operating on completely different assumptions. That is what makes this so painful: the rule you want to point to was left unwritten. Your hurt is real regardless.
In an undefined relationship, the shared line was never drawn, which can leave you doubting whether you are even entitled to the feeling. You are. A missing label can sit alongside a real understanding, and it certainly sits alongside real pain. It mattered to you long before any formal agreement could have.
Who was technically right is a question that rarely produces relief, because both of you can be right by your own unspoken rules. What stays open, and matters more, is what you actually need from this relationship, and whether it can give it.
Have the conversation that ends the ambiguity rather than the one that litigates the past: "Here is what I understood us to be. Here is what I need. Is that something you want?" The answer, whatever it is, is information you can act on.
Go deeper
Why Won’t They Commit? looks at what exclusivity means when nobody said it out loud, and how to have that conversation before it becomes an argument.