According to Robert Wright, these reasons why some women cheat;
1. Resource extraction- getting things from multiple partners increases resource security.
2. Dilemma of off springs- Mating with multiple partners leaves each male with the impression that he might be the father of a particular off spring. There's scientific correlation between male's kindness to kids and chances he might be their father.
3. Good genes- Some studies show that women are likely to cheat on their husband during ovulation, suggesting, that they target certain genes instead of resources.