Physiologically, the hormonal processes required for sexual reproduction are much, much more complex for a woman than it is for a man. Since sex drive is, in terms of evolutionary advantages, a strategy to optimise both reproductive success and a way to best ensure the survival of the offspring through pair-bonding behaviour, I think it's safe to say that the 'end game' of reproductive success has quite a bit to do with one's sexual conduct, and that since women's hormonal cycles are so much more nuanced in order to grow a baby, so may be women's sexuality in a more general sense. And to be fair, if we're talking purely in terms of physical/biological function, it makes sense for a male to be sexually available (ie have a higher sex drive) for whenever the female is able to become pregnant due to it being such a small window of opportunity.

Human sexuality is a 1000 piece jigsaw with tiny pieces that are overwhelming to look at and think about putting together- we're still figuring out the upper left corner. Culture, religion, scarcity of time/resources, personal stress levels and self-care practices, hormones, security... Everything plays a role. While I believe that our mentality plays the most important role in maintaining sexual interest in an LTR, that is a lot easier to do with your foot on the proverbial gas pedal, aka a pressing physical and emotional need to have sex. That's the root motivation of most of our desires here I think, however nuanced they become in the light of day.

It's almost impossible for us to separate fact from fiction for the larger population, but as an anecdote- I'm a mid-20's woman in a 5 year relationship, my physical drive has indeed dwindled in that time (used to be borderline hypersexual, now it's a lot more mellow) but my desire for my partner has honestly increased quite a bit over the years despite that. He can get me completely ready for him in mere seconds typically. Not sure yet how I feel about the strictly mono lifestyle, but monogamy has not put a block on my desire at all- in fact, the exclusivity gets me hot. I'll let you know how things are going in another 5 years.