Maybe this might work in a world where there aren’t any STDs and the possibility of children. Also, if it happened for whatever reason, the cheated on person deserves to know, because while “ taking a secret to the grave" is a romantic notion, rarely does it stay that way. People crack, the other party might feel terrible guilt and tell your unknowing spouse etc etc… then you are truly done for. For me, just tell me, im a grown adult and need ibformation to make choices about my future: I’d rather deal with the person you are than some idealised version you aren’t.