Entering a relationship, regardless of marriage or not, is one of sole commitment to each other and trust. The unwritten contract is formed as basis for relationship, which gets cemented when the two marry. So, any deviation from that is cheating, no matter how one tries to justify or deny it. Some will justify that in degrees of cheating, having an affair is more serious than punting, but all relative as all points to problem and breach in relationship.
The problem is that man has a primeval drive to hunt, experiment and conquer that is to deep rooted i.e. given the right lady, the right situation, no fear of getting caught, every man will succumb to his primeval instinct. To twist an old roman quote, " He saw, he conquered, he came". So a man has to fight this instinct each and every day, and will succumb if the instinct is triggered by a situation, by the man, the relationship with his women etc. The following article has an interesting list of triggers.
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