Great question, would anyone agree with the following?
In the beginning of a relationship it is very exciting...etc. So, sex is high priority ( for whatever the mental/biological reason,) so you "make" time for it...skip meals, go to sleep at 3 am, leave work early, etc. As time passes and the initial excitement tapers off, the priority of it changes and after a long day, you're both tired so instead of sex, you go to sleep, instead of sex, you sometimes need to put in a little extra with work, instead of sex, you really are starving hungry so you eat.
Then, a couple months/years pass and you think back to how much sex you use to have and fall prey to an idea that you are not longer in a good relationship/attractive/attracted to him/her, etc. Flip side is that you or her have put on weight, not right for each other, or whatever, but is that a true measure of you or them as a person, or a relationship?
With the pressures and time commitments a "modern" life requires (self imposed or not,) it seems that many pleasures get forgotten, but we don't judge ourselves by which was the last time I was on a swingset, or went down a slide with hands in the air yelling "Weeeeeee!"
Currently, I think we have a true misconception of sex, like a super power that we're not quite sure how to use, and then we abuse or mistreat out of ignorance.
Not sure.....