"Why do people cheat? Why is it so easy, so acceptable to cheat?" These interesting questions were asked by Steemian 
Rather than answering the questions directly, I am going to flip things around. Why was it that in Western countries over the years it became a dominant social norm to NOT cheat, and why has this norm faded to a shadow of its former self?
Dominant social norms, such as the one in question, do not materialize out of thin air. They evolved over generations because they served a human interest, and humans mostly act in their own interests.
The key factor was human biology. To take the romance away for a moment, humans, like other biological creatures, have a primordial drive to reproduce. Since women have the children and men do not, this created a natural role for each sex.
Human babies are extremely fragile and helpless. Unlike what is typical in nature where a baby animal matures reasonably quickly and is able to fend for itself after a short period of intensive care, a full 10 years after they are born a human is still a defenseless child. Only in its teen years does a human develop a reasonable amount of independent strength, reasoning and physical maturity to survive on its own. This is a problem for human mothers, since they traditionally spent the large majority of their adult life caring for their children. So, as the ones in charge of having the babies and taking care of the babies, mothers needed resources: lots of resources! Otherwise, they and their children would not survive.
That's where the man comes in. Of course, men also want to reproduce, but doing so does not take them out of commission for several decades. Therefore, they biologically have much more time to do other things.
So, a social bargain was constructed to give both men and women what they wanted and needed. Women offered their fertility to men and men offered their resources and labour to women. For this bargain to work, both men and women needed certain conditions to be met. Women, wanting strong, healthy children and the resources to raise them to maturity withheld their sexuality from men who couldn't provide what they needed. So, there has always been a very strong sexual preference for strong men with a high level of resources. That is a fact.
Men, on the other hand, needed to know that the children they were supporting and paying for were actually theirs. Men aren't stupid. You don't break your back and frequently risk your life (as was the case in the olden days) for some other dude's kids. That is why men demanded, in return for their labour, their women's chastity.
In the ideal situation both men and women got what they wanted and needed. Over time, at least in Western countries, the moral taboos of breaking this fundamental biological contract were severe. For centuries the husband and wife pair bond became essential to daily life and the progress of society. My description may not be flowery and romantic, but a lot of good things came out of this social contract and the related morality of not cheating. Not cheating became not only a positive thing, but it pretty much became a necessary thing. A cheater, if discovered would suffer a terrible fate. A cheating man would be ostracized by polite society and face violent reprisals from the family of the woman. For the woman, the fate was typically much worse.
Fast forward 1000 years of Western history and enter the great corrupter: the modern welfare state. Traditionally, you needed your family, friends and tribe to survive. Life was brutal and short if you did not have what you needed. Of course, many unsuccessful people perished, while others not only survived but flourished. Then along came some people with a new and interesting idea: what would happen if The State would step-in and help those in need?
It seemed like a good idea, and in the beginning it probably was. Many of the worst aspects of human poverty and suffering were abated in the West. Since the concept of the Welfare State appeared to be working, it was expanded, and expanded, and expanded, and expanded ad infinitum. That is where things went wrong. That is where a good idea turned into a corrupting monster.
While general poverty was definitely reduced, there were many unintended consequences of the Mega-Welfare State. I won't talk about all of them, but instead bring this back on topic to the unintended consequences on the fundamental biological contract between men and women that had been in place since ancient times.
For the women, the Mega-Welfare State has become a substitute husband. If she needs resources to raise her children, but doesn't have a man, the state will provide benefits, free school, healthcare, housing and money. Since the modern woman does not technically NEED a man for anything other than sex as long as she is willing to leach off the state, this has freed her from her sacred bond of chastity. The social stigma associated with sleeping around has, therefore, collapsed. Now a woman can have many sexual partners and her life will still be fine. If she is in a long term relationship and she cheats, she is no longer facing ostracism, starvation, societal expulsion and misery. Therefore, she can have her cake and eat it too.
Men are no better. They have also realized that they can get sexual access to women and not pay for it with dangerous or back-breaking labour for the rest of their lives. They too can now sleep around. They can be Players! And besides, modern women frequently choose the Bad Boy over the Nice Guy, so perhaps they are even incentivized to be dirty cheaters and rogues.
The modern Mega-Welfare State has enabled the very worst of human characteristics to replace something which was once noble, virtuous and good. The Devil comes to you not as an enemy, but as a friend. He gives you something: something which was once forbidden. He tells you it is okay: the old morality is just silly and out of date. Just take this one bite of the apple, it is delicious....