I think it depends entirely on why a person enters into a relationship. If they do so expecting that as one of the things they want from the relationship, then I guess it does become a fundamental.
On the other hand, I think it's fairly easy to do without if it's not as high on the priorities scale, and a number of people genuinely do value the togetherness in senses other than the physical. In these instances, I think that sex is able to be removed from the relationship without any serious impact upon its stability or longevity.
In response to the sex getting boring and moving on thing, again I guess that depends on why you entered into the relationship in the first place. If you were after a quick fix or something less than permanent then I guess maybe the role of sex in the relationship again becomes more significant, whereas people commiting for the long haul are (often?) more interested in other things.
Afterall, if you're planning to stay with someone for a long time, there's only so much of it you can spend having sex, and things would get awfully boring if there was nothing else of interest going on
(Again, maybe other people see it differently, this is just what I think)