I think it is certainly possible to be completely blindsided by a spouse. I imagine there are many people who had complete faith in their spouses and who have ended up betrayed.
However, I think you make an excellent point about not putting yourself in a situation where betrayal can occur. I've also been keeping up with the threads about men and women being friends and I am shocked a the number of people who say they are okay with their spouses having solo lunches with members of the opposite sex, commuting to work with them, going to movies with them- even traveling with them.

I think the people who say they completely trust their spouses in this situation are the ones who do, occasionally, get blindsided. Spousal trust is a wonderful thing, but based on the all-too-common stories we read and hear about, temptation frequently proves too much for people. I think we sometimes underestimate the power of physiology and overestimate our willpower. It is not just bad, untrustworthy people who cheat on their spouses. I absolutely despise betrayal and lose respect for those who commit it, but it doesn't mean those people are "bad." They just caved to temptation- probably like you said, because they allowed themselves to get into a situation that could lead to betrayal. How many times have cheaters said something like, "I didn't mean for anything to happen; it just happened"? It "just happened" because a person put himself in a situation where it COULD happen. Why would a person risk a marriage and family so he can have a solo lunch date or a cheaper commute?
That applies to the people who say they signed up for AM out of curiosity. Why on earth risk something so valuable as a marriage and/or family just to satisfy one's curiosity?
Right there with you on this issue.