I am of the personal opinion that life-long monogomy is purely a social construct, and NOT the way we were biologically evolved to be.
Developmental Anthropologists like Dr. Helen Fisher in her book 'The Anatomy of Love' have argued that we originally evolved to be what they characterise as 'serial monogomists'-- meaning that we ARE designed to be 'temporarily' monogomous with another person, and that the emotional attachment we call 'love' came about as an evolutionary adaptation to keep both partners 'loyal' to each other long enough to rear at least one child together, the idea being that the guys would stick around long enough to help provide for any children that were produced in the relationship long enough to get them past the infant and toddler stages and into early childhood. The emotion of 'romantic love' they argue was biologically designed to last approx 5-7 years, to keep each partner 'loyal' and 'committed' to the other. Around that time, the argument goes, each partner would move on to other partners, and enter into another monogomous relationship of approximately the same duration, long enough to rear and provide for any children produced into early childhood. Funny that there are so many classic jokes about what we call the '7-Year Itch'... Seems there is a scientific argument which lends some credence to that!
http://www.amazon.com/Anatomy-Love-N.../dp/0449908976
http://www.psychologytoday.com/blog/...he-7-year-itch
The argument also states that this was in the genetic interests of the human race at its earliest stages, when there weren't that many people on the planet, as it promoted genetic diversity-- it was better for there to be a greater 'mix' of genes than for the same man and woman to be popping out dozens of kids together!
I originally read this book nearly 10 years ago and have watched numerous interviews with Dr. Helen Fisher and similar documentaries with her and other thinkers on the topic, and I must say it has profoundly shaped my own views and expectations about human sexuality and relationships. I honestly don't ever really expect to fall so deeply in love with someone to thr point that I would stay emphatically that I will NEVER desire sex with another person, and I won't allow myself to feel guilty for having those urges/desires, not do/would I allow myself to feel (or be made to feel) guilty for having those feelings or acting on them. That's not to say that I won't 'settle down' some day, for companionship or otherwise-- I'm only noting that I will enter into such a relationship with my eyes open and aware of the fact that my feelings for that person inevitably WILL change in some ways, and that I will almost inevitably desire and/or seek sex from other women during that relationship.
We are DESIGNED to desire sex, and have (as males) been endowed with the biological urge to have sex with as many females as possible, despite whatever social arrangements (marriage or relationship) we might currently find ourselves in. In my personal view, seeing escorts is itself simply a social construct designed for married me to fulfil that very urge. Afterall, ehy do you think that they call this the world's 'oldest profession'?
http://en.wikipedia.org/wiki/Prostitution