Coming from experience of being a jackass to girls during high school and hanging around guys that cheated all the time, there could have been many things wrong.
They Want to Push the Limits
A lot of guys use the excuse, "I was having doubts about the relationship, so I did it to see if she really cared" or "I was drunk and do not remember." These are just lies. One of the main reasons most guys cheat is because they want to see how much they can get away with before they lose their girlfriend. There's always the instance that the girls will break up with them but take them right back. Which, in most cases, the guy will just claim they will "never do it again."
Another reason that guys may cheat is because they get bored with the relationship and want to explore other options. I am sure that most girls will hear the statement "I want to go on a break." or "I think we should hang out with our friends more and take time away from each other." These are just excuses they use so guys can mess around with other girls.
Scared of Commitment
Finally, a reason a lot of guys will not admit but it is true, is that they are scared to commit themselves. A lot of guys don't like the idea of getting too attached to one person in fear of getting hurt. So they cheat to push the girl away. That's probably the most common reason a guy will cheat on you.
But I'm not saying that this is true for everyone. In most situations, these are the most common excuses that my guy friends and I have used in the past. Girls, do not fall for these excuses!