I have to agree with what others have said. Sex is important in a closed relationship. Closed meaning it is just the two people, not bringing anyone else into the equation.
For many women sex is a way to express their feelings for their partner. It shows trust in that partner by allowing them to see the most intimate details. It is to bring the two closer in ways that are sacred, only shared between the two.
I recently remarried to a wonderful partner. We just today discussed our own sex life, because we are trying to get in sync with one another in that area to improve it. One of his complaints is that I do not seem to be as involved in it as he would like. To him this makes it seem as if I am doing it only for the act itself, and not to be a part of him emotionally. So, even for him sex is not just a mere act to, pardon the phrase, "get his nuts off." he sees it as a way to bring us closer together. These were basically his own words, even if I haven't quoted them exactly.
I realize now, by being with my new husband, that in all my other relationships sex was just the act. No true feelings involved. It was just the act to get each other off, nothing more. There were no great emotional connections like I have now with my new husband. I can see now this is because of the man, himself. The others treated it just as an act to get off, while my new husband views it as a way to emotionally bond us.
So, even when the actual act itself does not make us see fireworks, we would still like to consider it great sex, because of the emotional bonding that occurred in the process. I think that in long term relationship somehow people tend to lose that ability to bond emotionally, leaving sex as nothing more than the act to get off.