Their Family Has A History Of Cheating
Scientists say that most of our behavior is predetermined by our genes and the rest of the environment that we grew up in. If your partner grew up in a family where it was acceptable to cheat, then there is a high probability that they will take that for granted and cheat themselves.
This type of behavior is seen by them as something that is normal and accepted in the relationship and they will not understand when you tell them that cheating is not acceptable. When you grow up in such an environment your values and conceptions of how a proper relationship should look like become eschewed.
If it was understood that cheating represents something you do on a regular basis then you yourself become a cheater. What is more, you don’t even feel guilty about it. It is recommended to avoid having a relationship with someone who grew up in this abnormal family environment.