They Are Repeat Offenders
If someone cheated before it means that they are capable of doing it again. It is a good idea to find out before you enter a relationship with someone if they have cheated before. Some people just cannot resist the urge to cheat because it is so tempting.
If the opposite sex pays them a lot of attention and they act like they like the attention, then that could lead to cheating. This might even happen with people who have never cheated before. If someone hasn’t cheated it doesn’t mean that they are unable to cheat.
It means that the opportunity to cheat hasn’t been presented to them. When someone suddenly receives a lot of attention they might not know how to deal with it. Therefore they might consider cheating on you in the future.