Many believe this; however, if two people are not yet married, then “adultery” is not yet going on in the eyes of the Church.
If the member of the unmarried couple involved with the outside party is having sex with them or contemplating doing so, then that could be a serious sin of fornication or lust, as well as deception of the other member of the unmarried couple and a red flag that perhaps the marriage should not go forward. But in order to commit adultery, you need to be married. There are cases where Person A is “on a path to marriage” with Person B and realizes that it is a mistake and they really wish to be with Person C instead, or at the very least do not want to be married to Person B . In these cases, ending the engagement is not necessarily sinful, although it can be very upsetting to Person B.