I've come to realize questions that begin with the word "Why" are more often than not “rhetorical questions”. The majority of us believe there is NO answer that is going to cause us to "understand", "empathize", or "forgive" the betrayal. “Why” is just a "reflex question". Truth be told, we really don't care (why).
Having said that I believe there are 3 Basic cheaters.
“The Incessant Cheater” whose motto is “variety is the spice of life”. He loves to flirt and seduce women. Most likely he has never been faithful in any long-term relationship.
“The Unbelievable Opportunity Cheater”. He was not actually looking to cheat but an unbelievable opportunity fell into is lap. The woman may be “out of his league” (beautiful, famous, rich/powerful) or someone he has had a secret crush on. When the “once in a lifetime chance” presented itself in his mind to say “no” would have been the equivalent of throwing away a winning lottery ticket. His motto is “The best way to deal with temptation is to yield to it!”
“The Discontented Cheater”. Of the 3 basic types of cheaters this is the one who actually feels “justified” on some level. He sees his cheating as being the result of something his mate is not doing for him….(sexually, showing appreciation, respect, admiration, or she complains more than compliments and he feels taken for granted. Bottom line in his mind is he is “reacting” to how he is being treated.
Cheaters seek to hold onto what is good/comfortable in their primary relationship while addressing their other "needs" on the side. They lack the courage to walk away from a "known present" to pursue an "unknown future". Hope this helps.
One man's opinion! :-)