When one of my friends knew that he was travelling so often for work, she casually asked me if I was worried. I was at first confused by her question and answered,"Of course! I'm worried about his safety and whether he'll be tired, blah blah" before realizing what she really meant.
Hidden in the question was actually, "Am I worried he'll stray?"
Not that I know he will stray but surely there has got to be a reason before the straying starts. We're very open with this and talked about it before. And the one thing he told me was this. For a man to stray, there has got to be something's missing in the marriage. And that something is always sex. Period.
From a man's perspective, when a woman starts to tease and flirt or pardon my language, 'cock tease' a man, it would be very hard (NO PUN INTENDED, OKAY!) to reject that sort of attention. She doesn't even need to be extremely attractive, just pleasant and comfortable to talk with and add in just that bit of seduction whether intentionally or not, the man will fall for her.
And it's not just the man who'll always strays. For all you know, I might be the one who strays.
From a woman's perspective, when a man shows you attention in little ways like being a listening ear, always offering to help you in whatever ways he can, compliments you, just being there for you emotionally, little by little, the attachment will grow.
But I think for me personally, it stems from a loneliness in the marriage before all these can happen. Because it sure gets lonely in the hotel room with no one to talk to, and back at home, it sure gets lonely with no one to cuddle up with.