Did this casual relationship involve sex? If so, I believe all pertinent cards (such as the existence of other partners) should be on the table before starting a sexual relationship. I know timing doesn't always work that way, but it sure seems like she withheld info from you on purpose until she got "found out" and that's absolutely deceitful.
Even if the relationship didn't involve sex yet, the fact that it doesn't sit well with you is really all you need. You desire a level of honesty that she didn't provide. It may not have been intentionally dishonest, but that doesn't mean you have to automatically accept it and behave as though you don't have problem with her choices.
Pan Female, Hinge in a V between my mono (straight) husband, Monochrome and my poly (pan) partner, ThatGuyInBlack