Essentially, threadstarter is saying absence makes the heart grow fonder so spend more time apart and that will make the time you actually spend together more pleasant and precious with your significant other.
The reality is people aren't going to be sleeping in seperate bedrooms, going on vacations seperately, and doing all that just to "spice" things up.
For most people, especially men, some women too, it's easier just to get some p*ssy/d*ck on the side, keep it discreet and keep it moving.
Most people have either cheated or been involved with a cheater at some point. I'm not advocating cheating, but I understand a lot of the psychology behind it.
The curiosity of tapping into the unknown or supplimenting what you have is a temptation to much for many to bypass especially when given the opportunity of discretion and no one necessarily having to know other than the two parties involved.
Some people will say it's selfishness and lack of character, but I tend to believe it says more about discipline in that area and people not being able to solve their boredom or frustration in a situation more than anything. It doesn't necessarily mean the given person doesn't love their significant other either in my opinion.
Penises and vaginas engaging with one another has very little to do with love and more to do with innate desires and urges some can handle better than others.