Believe me, there are days when I wish I could take the average American who stumbles their way out of the US and flog them for being so ignorant. That is an entirely different subject.
It is a poor excuse to plead ignorance. I agree it is a reason, the same reason American students try showing up every year at UK unvirsities without proper visas, but the rest of the world going out of the way to coddle the poor Americans who can't type "visiting the UK" into google is sad and disrespectful to all of us who were able to figure it out. Again, I am willing to fight for 320(11) rejections, because I believe they are unfair in some cases, but as long as people have a way to formalise their relationships and get them into a situation where they can be assured that they are contributing to the society that I am now apart of, I cannot argue with the rules, even if they sometimes seem pendantic when applied in certain cases.
But, even if we agree such coddling is necessary and that the average American cannot take responsability for themselves, the OP and his fiancée obviously felt there was some need to "keep their stories straight" about lying to an IO. Out of ignorance, yes, was there malice, no, but whenever you proactively decide to stretch the truth it can and sometimes does come back to haunt you.
Now, I am going to get onto a touchy subject, which for the life of me, I can't figure out, why do young American women feel the need to come over to the UK and live with their boyfriends for extended periods of time as a visitor? A month, perfect, two months, fine, three months, how many times can you visit the Tate Modern, really, but 5 or 6 months and then leave and try to come back in for another long period of time? You have to be joking me. The "we can't bear to be apart" crap doesn't cut it with me. I went through 5 years of crap trying to live with my partner full-time, but the last thing I was going to do was just "hang out" for several months in his flat.
Do we think their might be somewhat of a reason why young single women from America might be targeted by IOs?