Can Someone Explain the Mark Sanford Mindset to Me?

The aspect of adultery that is so difficult for me to grasp, as laid out for us in graphic detail (TMI in fact) by the wandering gov, is why cheating spouses think it is OK to cheat.

Doesn’t this require enormous ego, hubris, rationalization?

Not the act, folks, but the idea that you can take whatever pleasure you want with someone else (soul mates, FGS) while lying to your spouse about it, thus denying the spouse the choice of whether she or he wants to stay signed on for this.