If you didn't know, 4getmenot, then how could that possibly make you a mug? Of course, if/when you do find out that your partner is cheating on you then you may well find that certain people come out of the woodwork telling you how they knew all along. Only then will you feel a mug.
Ignorance IS bliss, but it's not always a good thing, as exemplified here.
I just couldn't imagine being in either situation in reality. I'm far too inquisitive and nosey at the best of times so my partner would find it hard to cheat without me finding out in no time. My last relationship being a perfect example! I'm also a jealous person, but I would hate that to negatively impact my partner as I know that is my issue and not his. So I tend not to let any jealousy or trust issues affect me or him.