There is no absolute data available as to whether or not men have just one affair. However, there is a school of thought which believes that many people (women included!) who have had one affair, will have at least another.
This may well be because once the line of infidelity has been crossed, it is easier to cross it for the second time, so it is not such a big deal and therefore seems easier.
In addition, many people find having an affair quite exciting, the meeting someone knew, the butterflies in the stomach and feelings of lust and desire. These feelings can often subside during a long term relationship and so rekindling them can be exciting and an affair, at least in the early stages, can be exciting and a relief from the ordinary and mundane.
But that being said, many people do have an affair and then manage to rebuild their original relationship and no more infidelity occurs. It simply depends on the people involved.

 WE make too big a deal over this!! Both sexes are not normally monogamous. It is the society that is to blame. The person you loved at 16 is not the same at 30. We all grow and have different needs.