Its a lifeboat and you are in denial if you don't agree...
People don't keep former partners around just to chum around with... they want to have someone on hand to "talk to" if they need them...
Its laying the groundwork for a sleazy affair... this is how most affairs START... with friends whom you are attacted to that you keep secret contact with...
And yes, unless your wife reads your facebook account along with you its a secret contact.. even if the convo is at the moment apparently innocent.. its setting the foundation for an affair.
