Infidelity Support Group
Any relationship in which one partner engages willfully in sexual relations with another outside of the partnership is considered to have experienced infidelity. This breach of trust is often traumatizing for the faithful partner as well as the relationship, and support is often needed to heal emotionally and to decide whether or not the relationship should continue after...
sexual fulfillment
a confidant / friendship that may not exist with the spouse
The whole issue of being in the "fog" comes into play, too. WS may believe that they've finally just found the "right" person and that their spouse was just "wrong" for them. It may be a sense of glorifying the AP and making the BS into a villain. Sometimes it's more complicated... the sense of being trapped (blackmail, obligation, whatever) after things have gone even slightly inappropriate so a WS gets "stuck" in an affair. (Not to excuse the behavior, just some of the things I've seen play out.)
All I can say now is: What comes around, goes back around!!
BS=betrayed spouse