Hey FIB,
I have been saying this (OP) for a long time. It is now (actually has been for a while) obvious. Why would a spouse up and leave a marriage where the other spouse (although not perfect) is not abusing him or her? "I need space" = I need time to figure out if the other person (who I have been seeing for a while) is the real deal.
So in retrospect, when you hear that or the other lines including the "I love you but....", just get the hell out of there. Sure there are logistics involved and kids....but if you want to have a chance, that is the way to go. I guess going dark right away...but hey we may be a little tainted.