There is an excellent web site devoted to helping couples rebuild marriages that have suffered through infidelity.
Here's a link to the opening of that discussion. It's a good read for those interested enough in understanding why it happens and how you might actually overcome it.
For those seeking more elightenment there is also a substantial amount of information on how to make your marriage affair proof. Tool around the site...if this helps even one person/couple here then it was worth me posting it.
Marriage Builders