There are many reasons why people are unfaithful in their primary relationships. It is necessary to look deeper into why your mate or potential love interest has previously strayed before answering that question. Furthermore, women and men often differ in their motivation to cheat.
"Piggy-backing" is one of the primary reasons women start seeing another person. Unhappy with their primary relationship, but too dependent to go it alone, they cheat on their partner in the pursuit of a new partner.
Once a relationship with the new love interest has been secured, these women typically return to monogamous behavior. As long as the new relationship remains satisfying, they remain faithful.
Men who cheat, more frequently do so for sexual gratification. Many enjoy and pursue relationships that are essentially sexual outside of their primary relationship. However, there are also those who do fall in love with the other woman.
Within this last group, there are men who either won't or are unable to leave the primary relationship due to familial obligations. Or they also still love their primary partner and are comfortable with living a double life. These men will indefinitely maintain concurrent relationships and a state of infidelity. Other men follow their heart's desire and do leave their primary relationship to become fully involved with the new partner.
Just because a relationship starts while one or both partners are being unfaithful to someone does not automatically mean that the pattern will be repeated.
It also holds true that if someone has been unfaithful to you in the past, they are not necessarily going to betray you again, as long as the core issue that initially caused the cheating is resolved. Keep in mind, though, that some people are driven to cheat by unresolved personal issues that require more help than a loving partner.
Also, if you have a pattern of becoming involved with unavailable partners, then you might need a deeper exploration with a knowledgeable adviser to address your own issues.
What prevents a cheater from cheating again, is understanding and resolving the root cause of why the cheating occurred in the first place. Habitual cheaters do exist, but there are also those who truly want to be in a satisfying, monogamous relationship. A former cheater who does not have deeper issues driving his or her infidelity, and who is happy and satisfied, is unlikely to wander again.