I’ve studied this topic quite extensively (although being in a non-dead-bedroom relationship myself), and I'm confident in my claims – but I expect to be downvoted, as my thesis contradicts one of the fundamental pillars of society. (Text got a bit longer, sorry, but I need to elaborate a bit for it to make sense).

TL;DR: If you have high libido & sex is important to you, a typical long term monogamous relationship is one of the worst possible strategies (except maybe 1% who get lucky). Sexual patriarchy has concealed this historically, but today, it becomes apparent that long term monogamy is a surefire recipe for a bad and boring sex life (with women struggling more to keep their desire). There is no empirical evidence supporting the opposite

  • The “sexless marriage” phenomenon in entertainment and our direct environment is ubiquitous – however, most of us still assume “that won’t apply to me” (one of the most common cognitive biases)
  • It doesn’t help that church/entertainment/culture are strong forces which push the monogamy narrative, and media outlets marginalize or whitewash the issue (“here’s 5 ways to bring the fire back”, “just communicate with your partner, bro!”)
  • We generally accept that sex in a long term relationship becomes less frequent and exciting once the “new relationship energy” wears off. Additionally, the longer the relationship, the less inclined one is to try new things / experiment with kinks etc. (some people will vehemently disagree, but in general, you just have too much to lose if you gross out your partner of 20 years, that person you see every day and share a house and insurance and kids with, with some strange new fetish)
  • Sex frequency is negatively correlated with age and relationship duration, with self-reported 2-3 times sex a month average for long term couples. Gay couples have vastly higher intercourse frequency than hetero couples, lesbian couples have the lowest by far (about 50% having it once a month or less)
  • The last point hints at one central underlying problem (which is apparently not PC at all): biological differences between women and men (men having about 20x more testosterone than women and a vastly higher and more “urgent” base libido, also much higher masturbation frequency, which largely explains today’s casual dating experience (cf. Tinder experiments with women vs. men – it’s eye-opening))
  • At least a third of women experience low sexual desire, about 10% report being asexual. Men’s numbers are drastically lower
  • And yes, women can be very sexual – just not with their long term partners, typically. New research indicates that the “women are made for long term monogamy” narrative is plain wrong. In fact, women’s sexual desire (for their partner) plummets dramatically around the 2 year LTR mark, while men’s only declines gradually over the years (cf. Wednesday Martin’s book: “Untrue”, incl. studies, or "Women Get Bored with Long Term Relationships" in the Atlantic
  • I'm not denying men struggle with monogamy as well. However, women now have surpassed men in cheating for younger couples
  • Historically, this problem has been counteracted quite effectively by “sexual patriarchy”, i.e. a) men being able to “take sex” by force, legitimized by law & religion, whenever they want, b) high status men having multiple wives, or c) men (and sometimes women) being given a “pass” to cheat (still more common in countries like France. Less than 10% find this acceptable today)
  • As Western men lost these privileges, paired with women gaining equal rights and sexual freedom, enforced monogamy moves more and more towards a “winner takes all” sexual marketplace, shaped by female hypergamy (the top 5-10% of males getting 50-80% of the women with more casual dating, i.e. a growing portion of men remaining completely sexless – which current numbers confirm,the numbers of men having no sex at all doubling in 10 years, while women’s frequency remains constant)
  • While frequency declines in general, researchers were suprised to find that sex frequency decreased most in married couples, not singles
  • Trying to “fix” the dead bedroom mostly only makes it worse (there are almost no success stories on r/deadbedrooms). The low-libido partner typically feels pressured, furthering his or her excuses and avoidance strategies (often times leading to physical repulsion to advances by the SO), while the high-libido partner becomes more and more frustrated and resentful towards the SO
  • My estimation (please don’t pin me down on the %s): about 90% of monogamous couples are sexually dissatisfied (libido mismatch, boredom, duty sex etc.). Among the 10% which aren't, 9% are (or have become) asexual and have no sex (and don’t care anymore). Of the dissatisfied 90%, about half cheat and/or leave the relationship, the other half slowly “come to terms” with their bad sex lives (status quo bias is incredibly strong in humans- this is probably your parents and most couples you know). That leaves only 1% of couples that are sexually satisfied consistently
  • The 1% who “make it work” (not with duty sex) are outliers, unicorns (which should be studied further; possible reasons could be hypersexuality of the woman, couples who live apart with no kids which avoids complacency, or the male is much more attractive or high status and the latent “dread” keeps the woman interested)

Summary: You can try to square a circle over and over by stubbornly following what the mainstream tells you (long term relationship = consistent good sex), or you can accept evidence and reason (obviously, being an incel who never leaves his basement is an even worse strategy for sexual fulfillment)

P.S.: This post is in no way intended to push misogyny or “x gender is better than y” – my goal is to question the current narrative and maybe help you make better life decisions. And yes, there are plenty other valid reasons to be in a long-term relationship other than sex