Being in pastoral ministry for almost 13 years now has put me in the tough spot of being pastorally involved in more than a few cases of adultery. Tim Challies has written as clearly as anyone on how an affair really begins. I’ve summarized this below, but wholeheartedly recommend reading his longer version.
Affairs begin when affection in your marriage is eliminated. When you stop pursuing your spouse, you’ll start pursuing someone else.
As intimacy in your marriage is eliminated, you’ll eventually encounter someone else who is attractive to you. There will be something about that other person that draws you in. You may begin to see promises of something lacking in your marriage.
After the initial encounter with that other person, you’ll begin to enjoy being with that person. By enjoying that other person, you’ve now given them an emotional space previously reserved for your spouse.
Enjoyment of the other person will inevitably lead to expediting opportunities to be with them: arranging for your paths to cross; lingering where that person is likely to be; looking for opportunities to text or talk.
At this point, the emotional connection is so strong it will invariably lead to verbal expressions of it: “I really enjoy being with you.” “I wish I could talk with my husband/wife like I’m able to talk with you.” Once the emotional bond is forged, it's only a matter of time before the physical bond follows suit.
This the culmination of everything that’s taken place beforehand. Now the two will find themselves in bed as adulterers experiencing physically what they already established emotionally.
The 17th century Puritan Pastor, John Owen, was right: the smallest sin is but one step to the biggest and most treacherous sin. That one small step of neglecting to pursue your spouse is the first sin which lets loose an avalanche of destruction.