Permit me to disagree with exactly this part of what you wrote. There are people who are capable of being faithful in the face of temptation. But if you do go out looking for temptation, you will find it.
Both Rossini and Vrabel were disgusting people long before they went and proved it to the world. Cheaters look for an excuse to cheat, none of them wait for the proverbial gun to the head. And they were already this kind of person that might do this long before they met each other. This incident just proved their true nature to the world.
Sadly you are right, this isn't uncommon. But I don't want to let the idea stand that it's somehow tolerable to be this kind of human just because it happens more than we like to think it does. Morality aside, this behavior creates a generation of kids that don't know their own genetic history due to kept secrets, and there you find the true victims of these things.
It's more than just the temptation, it's an extremely complicated behavior and many, many "good" people have strayed from their relationships and many, many will in the future.
Of course, many people who take this course of action are, in fact, not good people, but morality simply isn't that black and white.
You enter into a relationship with the understanding that practically speaking, there's a possibility your partner - or you - will not remain faithful.
Just as a drug addict wouldn't be a drug addict if certain variables had broken differently, a cheater wouldn't be a cheater if certain variables had broken differently. Every situation must be viewed in context. Unfortunately (or probably fortunately), we typically aren't privy to that context. That's why it's silly to judge people as human beings based on whether or not they're faithful in their relationships.
Some people cheat in a given relationship and then never cheat again in subsequent relationships. Some people are immaculately faithful for years, the relationship ends and then they cheat in the next one. Are we to condemn them for that?
And indeed, some people simply never stray. I suppose that's admirable, but I wouldn't assume someone is a good person because they don't cheat on their significant other and in turn, I wouldn't assume they were a bad person if they do.
But you're right, I'm sure there are *some* people who would never cheat under any circumstances. I just don't think that makes them any more righteous than anyone else.