In my experience... And maybe other folk in long term relationships can get on board with this, maybe not... But relationships are at least 30% CHOOSING commitment and love. Sure there's new and exciting, maybe even more attractive, pussy and dick out there. But you kinda just balance things out and figure alright, this person has been pretty awesome, with no awful surprises. Why risk losing them over a grass-is-greener mentality?
I think a lot of people have a fairy tale idea that happy couples get butterflies every time they look at each other or something. You kinda just choose a person who has stood out from the masses, follow the rules of loyalty and respect, and seriously grow into a happier person because of it.
Just my ten cents. My boss was bitching about being single and how she wants a dude who always makes her feel giddy. I learned that giddiness is due to constant uncertainty a long time ago, shocked that at age 40 she still hasn't.