Is that really true SGB, that you know more people who flap and whine and snoop than you know people who are happy and trusting in their monogamous relationships? It's the opposite for me. I know far more people who don't worry about infidelity at all, apart from on MN of course, where we're more likely to encounter the fearful and worried.
Secunda, I would echo the advice about don't set your bar so low. There are lots of people who won't cheat, but I do think couples should have a conversation about fidelity and monogamy and outline their expectations, rather than as SGB suggests, assuming fidelity - even in a marriage, where perhaps it is explicit.
However, I do think realism has to set in about how someone might feel after being in a long relationship that is ticking along just fine - and someone comes along and declares their attraction. It can be a very heady experience and IME, people always under-estimate how intoxicating it can be feeling desired and wanted with such intensity after such a long time. It is in this area that I think there is more potential for infidelity these days, as well as friendships that cross the line.
I also think that social networking sites increase the likelihood that old flames will get in touch and I see a marked increase in doomed affairs between people convinced that they were eachother's long lost soul mate, conveniently forgetting that the relationship failed first time round for perfectly valid reasons.
I think the relationships at the greatest risk are the ones where no realistic conversations take place about how one or both of the parties will respond to a sure-fire opportunity, where there is minimal risk of discovery. I'm somewhat cynical about the 35% who haven't cheated I'm afraid, because I think that a fair proportion of those people have simply never had the opportunity, or it was just too risky, so their fidelity hasn't been tested in a meaningful way.
However, I absolutely believe that there is a percentage of people of both sexes who would never cheat, regardless of opportunity or even if their marriages are not blissfully happy. Their reasons for not cheating are usually about a myriad of factors; their love and respect, their commitment to their marriage and/or the children - and their moral code.
There's an interesting interview in the Not Just Friends book where a man explains that on good days, he is committed to his wife, on average days he is committed to his marriage/family and on bad days, he is committed to his commitment.
I have concluded that the common denominator in terms of personality traits amongst all the non-cheaters is unselfishness. This is not to be confused however with low self esteem, because often the people with the highest self-esteem are also the most unselfish of individuals.
My bar is set very high and having gone through the pain of infidelity once, I would never forgive infidelity in any form again. There are no secret bargains about what would be forgiveable or acceptable, it is explicit and non-negotiable. Our boundaries about what is acceptable are crystal clear and shared by us both. It wasn't always so - and contrary to what one might expect in the aftermath of an affair, we both feel much safer and secure now that those boundaries are so clear and the fortress around our marriage, this strong and impenetrable.
So I would say, have high expectations, choose a partner who is on the whole unselfish but with high self-esteem (and therefore less vulnerable to an ego-stroke) - and be realistic about how you will both deal with possible tempations. Be clear from the outset what behaviour is acceptable and what would cause you discomfort. Never assume what your partner might do if offered an opportunity and don't feel scared about admitting your vulnerability - don't pretend to be "cool" with threats that make you feel uneasy.
Infidelity is not inevitable at all, but it is as you say, too commonplace as to make it absurd to cross one's fingers and hope it will never happen.