Is it true that it's natural for married men to seek for other women's company?

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Once my friend was driving on her way home, when she saw her husband's car, right about to pass her by. When she saw him he was smiling and laughing with another woman inside the car. And according to her she knew that woman wasn't a business partner of some sort. Anyways when my friend asked her husband about the mysterious girl. He said it was the girlfriend of his friend and that his friend was at the back seat. My friend said, "Who would let his girlfriend sit in the front seat with his friend?" The husband couldn't answer and he obviously had this guilty look on his face. My friend told him that once he's caught like that again she;s leaving him with their children. Anyways, when we were talking about it she was laughing and making fun of him. And she said that at one point of a marriage its natural for your husband to cheat on you physically, but in the end the commitment still remains to the wife, she also added, "He's a guy and guys are weak to temptation. When someone flirts with them, they can't help but flirt back." For me, it is an inexcusable thing to say that guys are weak to temptation and that once you promised to commit till death do you part, you shouldn't go wandering off looking for someone to please your physical needs. Well, that's just me. What about you guys? I need women and men's opinion on this.