Sen. John Ensign carried on what?

Whatever it was, it was one of a kind, apparently, if this article is a witness:

Republican Sen. John Ensign of Nevada has stepped down from his leadership post one day after admitting he carried on a marital affair with a woman who was on his campaign staff.

Sent me to a dictionary, but in vain. Why should a person be punished for a marital affair, anyway? I am having one for more years than they give you for offing someone, by the way, and really, there is absolutely no call for punishment, I swear. So, can I have at least some extra anchovies with it?