I generally believe that when someone has an affair, it's their issue. It's sad for the spouse and family affected, but it's basically personal. I understand that when it involves a public figure it's news (see Edwards, John), and I'm fine with that, but I wouldn't necessarily feel the need to comment on it.
Here is candidate Ensign in 1998 suggesting that Bill Clinton resign because of his affair, saying that he "had no credibility left" (how's the credibility thing working out for you?)
Here is Sen. Ensign explaining in 2004 that we need a Constitutional amendment protecting marriage from Teh Gay (I'm guessing that's why he had an affair, because LGBT people love each other).
Here is Sen. Ensign in 2007 explaining why Larry Craig should resign ""There's too many people that paint with a broad brush that we're all corrupt, we're all amoral. … And having these kinds of things happen, whether it's a Republican or Democratic senator — we certainly have had plenty of Democratic scandals in the past — we need people who are in office who will hold themselves to a little higher standard." (that one pretty much speaks for itself)
Listen Senator - You take care of your own house before you go telling everyone else how theirs ought to be built...