I think the discussion has missed some fundamental points. Sex is often hard to maintain in a long-term relationship because sex is in many ways fuelled by the opposite of love. Yes good sex feeds into maintaining love, but paradoxically what makes sex happen is desire, and desire is driven by a sense of the unknown. Love is about support, stability, reliability and these things undermine desire. So it often breaks down.
A majority of people in studies on LTRs report that the times they feel most attracted to their partner are when they see them "holding court"; doing what they are best at. Their work, their passions, their hobbies etc. Excelling at something. And this reintroduces the "otherness" and reminds them why they fell in love with that person and reintroduces attraction so that sex is desired, not forced and boring and routine.
A poster describing it as similar to exercise regimes is so depressing. It should never seem like a chore. Both partners need to put in the effort to continue to make themselves desirable to the other one, so that sex is desired on both sides. Otherwise it is doomed to failure.
Time apart, having your own jobs/ interests/ hobbies/ friends. Not being under each other's feet 24/7 and actually having time and space to miss rather than irritate each other, are so important to keep a relationship going. Flames need fuel and oxygen to stay alive.
The idea of scheduling sex fundamentally undermines the way desire works, the idea of the unknown, or excitement. Far better to surprise someone than schedule it in a diary... it's no coincidence that couples who struggle with fertility and get into a cycle of having to schedule sex for more fertile times often have relationship breakdowns (obviously much other stress and sadness at play too, but it has an impact). Anything on your "to do" list is never going to feel like fun, just another chore. It is such a destructive idea, for a majority of people (it may work for some but they are the outliers based on the research on this).
Good sex does help to maintain a relationship and feed into intimacy and love but for most people a long-term satisfying sex life won't be achieved by scheduled sex because that is the death of desire and spontaneity, and therefore for each person feeling genuinely desired. Going through the motions is no substitute, even if it ends up being physically enjoyable once you start. It may work as a short term way to keep things going through tough times like when you have tiny children, but not as a permanent way to live.