I will honestly say that I have cheated before. It's not something that I am proud of. I was younger and didn't value myself or my relationship enough to consider the decision I was making. It is better to end a relationship than to cheat. If you are even considering cheating then there must be something lacking in your relationship that is not able to be fixed or you don't care enough about the person and should let them move on to find someone who does.
And I do think there are more people that would rather not admit to it.

That and I'm sure their survey was done on a larger scale than a website forum. A true survey can't come from a questionnaire on the internet...