I think if there's opportunity, most people will cheat. Some people are simply selfish and as long as there's is someone who makes themselves available to the partner in that way (physical, emotional, etc.) the person will cheat. Some don't care if they're caught while others will cover it up for as long as they can. There have been a variety of studies that show that man is not monogamous (this goes for many "ladies" as well unfortunately) and it is a small number of couples that last for decades or until death, in a monogamous relationship. If this evidence doesn't sway you, look at the rates of divorce and cheating and choose your partner wisely.