Maybe it's because the other individual that they are dating is lacking a character that a women/man seeks and the person is incompetent to desire them. This applies to married and dating couples, another reason why they might cheat is because the relationship could be too restrictive, it's normal for men and women to flirt when around the opposite sex but we see this as a bad sign that the person in the relationship isn't as committed to the person they are subjective to. This could lead to both off the individuals to restrict themselves from certain activities that could make them flirt with anyone and as this piles up one of the two will cheat to get what they want.