Chris Rock says a man is only as faithful as his options. LOL.
I think, to add to @Jleslie’s list, some people of all genders are impulsive. They see food they want, they eat it, they see a person they want sexually, they go for it.
I don’t consider an open relationship, as mentioned by someone previously, as cheating. Just my opinion – if the spouse gives permission, then I don’t view it as cheating.
I think (again, just my opinion) that sometimes marriages and long relationships lose the spark, so people cheat because they want the spark back. The wife is tired of the hubby, the hubby is tired of the wife, they fight with each other, whatever, and then the hubby sees a young thing who is attentive and flirty and wham-o, that’s all she wrote.