I think there are a kabillion reasons aka excuses for infidelity. There's weakness...in my little world, you make a committment to your partner but it is the committment to MYSELF that would prevent me from infidelity. So,bottom line, I do think infidelity is a weakness of character, but then I am opinionated!
There are serial adulterers ...and we've read about many a one here on the boards and that seems to be a whole new beast.
But I think that that people who justify their own behaviors by blaming others are most susceptible to giving in to affairs. Unfortunately, there really aren't always signs of disaffection of spouse...because some people just don't seem to see the harm in it. (Pen, you might have the French background to harken to re les affaires des cours (sp?). I KNEW my X was depressed and hurt and angry but I was just tired of rescuing him...damn, I wanted to be rescued for a change. Instead he found someone eager to rescue him. Oh well. But I've so many friends whose spouses just decided a fling was in order...that they somehow deserved it???????? I don't think you can protect against it, but you can try to keep the white elephants out of the living room. If you can face the issues as they come up, chances are you can keep them from taking over the love...and then there is that LOYALTY GENE, but that is another story gd