If you are serious about guarding your marriage, you cannot ignore the one common denominator of just about every infidelity. In the stories I hear from the imploding marriages I counsel, it comes up every single time.
It is the place where the little sparks of flirtation become the flames of intimacy and eventually grow into the roaring bonfire of adultery. (Please, don't try these metaphors at home. I'm a professional.)
What is the genesis of most affairs in this day and age? You'll find it here.
Be wise enough not to ignore it. Love your spouse and value your marriage enough to diligently protect it.