Like I said, not everyone would be susceptible to "straying". However, I believe it's naive to think that as long as there's nothing missing in a relationship then there's no danger of emotional or physical infidelity. Everyone's emotional make-up, thought processes, commitment levels, etc. are different, so you can't put everyone in the same box.
My point is that because we are human, and because we are falliable, is it really worth it to potentially put a relationship at risk?
In our minds, at least, it may be possible to love more than one person the way we love a spouse. And while there doesn't appear to be anything missing from a relationship, if you experience something (or someone) new, then you can suddenly think to yourself "look what I've been missing, I'd really love to have that (or him, or her)".
Guys might be able to relate to this analogy. You finally got your dream car. You absolutely love it and have no complaints. A few years later your neighbor get his dream car. He lets you come over and see it, even lets you drive it a few times. While you still love your car, there's just something about this new one that "gets your motor running". I'm not intending to demean relationships with a car analogy (nobody gets hurt when we're talking about cars), but I do think the theory holds water.
But the real question is, is it worth the potential risk?
As a disclaimer, I should probably say that my POV is biblical (evangelical Christian), which in my experience puts me in the minority. I don't say this to judge anyone, but to provide a basis for my position.