Relationships built on sex don't last.
I've learned a few things about love in my time here on Earth, and one of them is that relationships built on sex usually don't last. First, no matter how cool she is, no matter how good-looking she is and no matter how much you dig her, there is someone out there who is sick of her. (This probably applies to guys as well.) Second, there is a very good chance that when a guy begins drifting away from a lady, he's just sick of boning her. (Again, it's possible that ladies stray for the same reason, but being a guy, I really can't be sure.)
Shocking, no? But it happens. Oftentimes, a relationship will begin on the concept of mutual attraction, loneliness or general horniness. A couple of kooky kids will decide they really like making sweet lust to each other even though they don't have that much in common. After a while, the sex will go stale and there's only so many times you can say, "Did you read what Thomas Friedman said today about the economy/environment/Middle East peace process?"
The film Speed ends with the Keanu character telling the Sandra Bullock character that a relationship can't survive if it's based on a traumatic moment, so she proposes to make it about sex…and he's replaced by Jason Patric in the sequel. Coincidence? Contract dispute? Or was his character just sick of sexing up her character?
Many people advocate waiting a bit before hopping into bed with someone.
The conventional wisdom is that this prevents disease and makes men stick around longer. My friends Matt & Tamsen suggest holding off 90 days before working on the night moves. While that may seem a bit extreme, even three months of bed-breakingly love-jonesing courtship doesn't guarantee you won't drift apart when the sex goes bland. But it does keep you from adding another notch to your belt from someone who won't work out.
One of my best homeys thinks that the point in a relationship when you're not having it off a dozen times per week or regularly sneaking into public washrooms together is when you really get to know each other. True though that may be, the Lycanthropic lust that drove you together might have been the only glue that bound you. At some point, curiosity and opportunity can overwhelm oxytocin, and someone will stray. And it's not just sex addicts.
Here's an example: a guy meets a terribly attractive nanny from Norland. This man is at the top of his field and possibly of mixed ethnicity. A colleague introduces them and it's lust at first sight. From there, they get married, pump out a couple kids and enjoy immense financial success. But the driven man possesses a substantial ego and wants to scale higher mountains and mount higher scallions and maybe the former nanny isn't as interesting now that she, ironically, is caring for kids that they made together. Maybe he's not super-keen to hearing about childrearing, interior design and fjords. Perhaps she isn't as turned on by green jackets and Buicks as she once was. So, he finds a string of women who are still excited by his drive and even though they can't hold a candle to his wife in terms of looks, he digs the novelty of fresh tail.
In no way does this excuse infidelity, but it ought to illuminate why certain relationships can't survive with foundations built on flesh.