I posted some of these thoughts when _refugee_ requested that I comment on the topic of cheating a couple of weeks ago.
There is much to fear when one engages in either open relationships or secret affairs. A big problem is that many people tend to fall "in love" in bed. Sex is a powerful vehicle for connection and communication.
Humans have so many needs -- emotional, psychological, physical, material, spiritual and needs we don't even know we have.
If there are any deficits in one's primary relationship, the deficits become magnified when one is feeling accepted, loved, and appreciated by someone other than one's spouse.
But the minefield of complicated relationships can be traversed successfully. While it's not for everyone, I increasingly meet people who amaze me in their navigational skills.