I have a theory. Bear with me because it might sound wacky.
Regardless of your moral programming, it is impossible to deny that humans have an inclination toward polygamy. To combat these urges, we have a fully-operational prefrontal cortex that enables the executive functioning to weigh the pros and cons of such misadventure. Still, many attached humans fall into the animalistic trap of chasing multiple partners. Others don't. Why don't they?
I have been happily married for a long time and would never pursue anything outside of the relationship. Long ago, I shut off that part of my brain and invested in my long-term relationship. Besides my choice to commit, what keeps me interested?
In any relationship, newness has value. Remember the butterflies you experienced when you first met your significant other? That's your caveman brain telling you, "here's an interesting female you've never copulated with - I'm going to flood your body with hormones so you get down to business".
Eventually those feelings subside - usually after nature has accomplished its mission to create and raise a child past the toddler years. Perfectly correlated with the infamous 'seven year itch'.
Not everyone succumbs to their instincts to spread their seed, but not without struggle. There are a plethora of advice columns explaining how to keep the spark alive. They tell people to change their routines, take up a new hobby together or communicate more.
I wonder if the simple answer is to change clothes. I believe that superficial changes in a spouse can temper the lizard brain's desire to find new and exciting mates. In other words, change the hair or the clothes, but don't change the person. Clothes become a proxy for polygamy because they re-introduce newness into the relationship, thus pacifying the lizard brain's primal urges.
Of course, for this to be true one must assume that humans were a lot more polygamous before clothes were invented. There are some theories that this was indeed the case for evolutionary purposes. I won't pretend to answer this question.
Perhaps I'm spewing nonsense. All I know is that when my wife changes her look in some way, I get that sparkle in my eye.