Relationships are like the blockchain.
There's no trusted, central database of all relationships that determines what is or isn't cheating.
That trust is negotiated in each interaction.
Marriages follow the traditional method of a chain of trust. There is a central database that lies with the government or the church, and everyone decides which database they want to trust.
You must have an interesting social life that every time you hook up, all past relationships immediately contact each other and vote on whether you are a cheater.