Curt at Happy Husband linked to us and caught a little flack over all of this talk about infidelity. He responded with his ideas in his post Monogomy vs Infidelity. He details his views on the high value of marriage and monogomy. I encourage you to go and read it.
I see the struggle with monogomy as a mirror to our struggle to worship only one God. Idolatry comes in many forms for all of us. As much as we want to love just our one God the truth is we find life apart from Him in lots of other gods. These other gods can be anything we find life in. Jobs, blogs, booze, drugs, football, kids, money, sex, relationships, affairs, porn, volunteering at church, golf, reading, any favorite sport, a flat stomach, nice clothes, ministry etc. can become gods when we define life by having 'it'. Think about taking 'it' away and if you gasp you have identified an idol. It can also be more subtle.
So it is with ones spouse. I love my wife and don't want to have sex or emotionally connect with another woman beyond what is appropriate in a marriage honoring friendship. And yet God did not give me a switch on my hip to turn off my attraction to other women once I put this ring on my finger. So monogomy is a daily, moment by moment decision that I make. To desire a monogomous marriage I am required to believe that God has my best interests at heart. And as much as I believe my God is good I still pray daily in some form, "Lord, I believe. Help my unbelief."