Love enjoys knowing everything about you; desire needs mystery.
We know desire is rooted in absence and yearning. What you don't have is often ten times richer than what you actually experience. An affair is a perfect erotic plot because it fits the erotic equation of psychotherapist Jack Morin: "Attraction plus obstacle equals excitement.".
In desire, there must be some small amount of tension. And that tension comes with the unknown, the unpredictable. You can close yourself off at home and say, "Whew, at last I'm in a place where I don't have to worry," or you can keep yourself open to the mystery and elusiveness of your partner.
The secret to desire in a long-term relationship
Even a good marriage leaves people with longings for certain things their marriage will never be. So, do they accept that, make compromises, and say, "You can't have everything in life," which is what we always did? Or do they say, "I deserve more. I want to experience that thing and, you know, I have fifty more years to live than I used to." It's not necessarily that we have more desires today, but we do feel more entitled to pursue them. We live in this "right to happiness" culture, and yes, we do live half a century longer than we used to.