I was MLT/MLC, but never had anyone else (no EA or PA).
Now it appears that my W is MLC without an OP (as far as I can tell, I still wonder as it would make some sense).
Still, most often there is someone else. A little snooping can usually determine if there is (cell phone bills, computer keylogger, etc).
If you find evidence do some reading on how to handle it. Some say call them on it immediately (i.e. Dobson's Tough Love approach), others simply stop snooping that point and wait for the affair to run its course.
Personally, I would go the Tough Love route, but that is up to you if and when you are faced with that choice.
