Some very interesting responses here, especially by sstype.
I'm in a LTR at the moment and I'd be lying if I said I have had no temptations. After nearly 3 years neither of us have drifted even once. My partner and I are very open, and we are very honest with each other-- to the point where she is able to ask me if i thought so and so was attractive and I'd give her an honest answer. This is a topic I will discuss with her in the near future.
I think many problems that people have is their lack of social boundaries and the word "privacy" in whilst in a committed relationship. If you are in a committed relationship these are things that must be discussed. Privacy is a word often thrown around by people who have something to hide, or want to maintain an "escape route" from their partner. There is a difference between "i'm going to take a sh*t, give me some privacy" and "No you can't see my emails, give me privacy". I'm not suggesting that snooping around is something you should do, but your email inbox should not be off limits or some sort of secret.
Some people are fine with having friends of the opposite sex. While this not wrong, there should be an understanding that hanging alone or with a group of the opposite sex is probably not something you should be doing. Actively searching for friends of the opp sex and allowing yourself to be open to approaches or sending signals of interest (or returning them) is definitely feeding temptations.
Having said that, if a relationship is going well then no amount of temptation by any level of attractiveness should be able to lead a man or woman to infidelity. A real man who is with a real woman should be able to construct simply thought out defenses to protect the otherwise successful relationship against intruders. So I do not agree with your quote. As long as I trust my woman, no amount of temptation would sway me---and many temptations would not come our way by means of our "defenses"
@sstype : those people would be successful whether or not they cheated. Tiger Woods would still be a pro golfer whether he was single or had been faithful to his wife. These "top" men are in those positions for reasons beyond their ability to fight temptation. Society is not rewarding them based on their ability to commit to a relationship. Interesting point of view nonetheless.