Let's start with the basics of cheating:
It seems like everyone is having affairs these days. You constantly hear in the news about famous people being caught or their mistresses coming forward or the man apologizing to his wife for cheating. The bigger reality is that there are more than likely hundreds of thousands of men across America that are cheating on their spouses at this very moment and they will not make the news. They are doctors, lawyers, accountants, and your average hardworking American, and they know what they are doing and able to keep it hidden from you. The majority of marital affairs are being instigated by the male portion. While females will cheat on their partners, they are much less likely to engage in that kind of behavior.
When a couple gets married, they say their marriage vows and promise, "until death do us part." The interesting part is that a vow of monogamy is not usually, if ever, part of a marriage vow.
Spouses cheat on their partners and have an affair for any number of reasons. It should be noted that the reasons men have an affair are generally different than the reasons females have an affair. Typically speaking the #1 reasons that men look to have an affair are sex and boredom. These reasons are completely different than the two main reasons women choose to have an affair. The #1 reasons that women resort to cheating are emotional reasons and financial reasons. When one of the parties in a marriage is not getting what they need in their relationship, they will seek what they want outside of their relationship.
There is many places that that initial spark for an affair can occur. Today, affairs are just much easier to engage in because of the Internet. The ability to have an affair is at one's fingertips... literally. Men are increasingly impulsive and compulsive and lacking self-discipline. If an affair is taking place, you can pretty much be sure that the Internet and technology is being used to keep the affair going.
How did the affair start is slightly different than the question on why it began. How it began was more than likely with several websites and in all likelihood out of curiosity. Because this site is designed towards women and men make up the majority of cheaters, we will focus on things from a man's perspective. He likely started looking through websites such as craigslist and maybe even Adult Friend Finder. He likely would tell you he was checking them out just to see what was out there, out of curiosity. In the back of his mind, he is ready to cross that line and checking to see where he could go to cross it.
A real man doesn't love a million girls. He loves one girl in a million ways.