This is my two pences on sexless marriage (drawn from my onw experience so others ight see it differently).
I think there are main 3 reasosn why a relationship ends up sexless.
1- Illness, whether physical or MH that will impact the wish of one partner to have sex - aka low libido.
I would include there any hormonal issues such as peri menopause or low testosterone.
I fell that hormonal issues are hard because people don't feel the need or wish to have sex at all so are unlikely to actually want to be treated for it because they are quite content with the situation. Also (again my own experience), going to see a GP migth not help a lot if they don't actually do the right test. I have low libido due to low testosterone level (yes even as a woman). My GP would never have checked for that and I only learnt about it because I was doing checks for other issues....
It is somehow different when people do have a sex drive but can't have intercourse (eg men and diabetes). They are more likely to seek medical support from a GP.
2- Lifestyle issues.
That's stress and working too hard - aka burn out. This is from the guy who works 60 hours a week, burns the caddle by both ends etc... to the woman who is running herself down to the ground because her partner is refusing to take on his share of the work (HW, parenting etc...)
In that case, people are basicaly so run down and/or stressed that sex is at the bottom of their list.
3- Relationship issues.
In that case, no sex is basically a symptom of a much wider issue. People end up not wanting to have sex (often not linked to low libido at all - so they might well still masturbate, watch porn etc...) becaue they are not attracted to their partner anymore.
My experience is that it is often linked to a lack of intimacy (not sexual) and closeness in the couple. Issues with communication, ressentment that has build up over the years etc...
I think points 1 and 2 often happen. But point 3 is the thing that is the break or make issue. You can have intimacy wo having sex. You can be great friends. You can work around illness, work issues etc...You can have a fulfilling life wo sex but lots of intimacy but I feel it's much harder to be happy if you only have perfunctory sex.
As @Macaroni46 said, they key is open communication. I'd say that if open communication on that subject doesn't happen and the (higher sex drive) partner is leaving then It's not about sex. It's a relationhsip issue with no communication and different life goals/expectations. That's the deal breaker.