One source I like pointing out is the now-defunct blog...
I'm still sounds very much to me like that statistic comes from nowhere in particular and is substantially "made up".
No, because it's the same everywhere and concurrent with economic development ie men no longer being selected for traditional provider roles. Many men simply can't compensate with other qualities which is the reason why there are more and more incels or men who have otherwise 'given up' in western countries or emerging economies.
Not necessarily.
Firstly, there's a difference between "can't compensate" and "haven't compensated". With new-found economic freedom, women perhaps do no longer need men as providers in the way they once did. So have men adjusted to this new social reality, or are many still stuck with slightly quaint social attitudes that are no longer best practice?
In conjunction with that, let's also consider the extent which things like "inceldom" may be a self-fulfilling state. Rather than tackle the issues about themselves which impede their ability to form a relationship, men instead are joining movements that encourage them to self-sabotage (PUA) or give up (incel) forming relationships, finding satisfaction of a sort by explaining their problem, rather than overcoming it.
It could actually be dating apps. Evidence suggests they are wildly inefficient and ineffective. What if young people have been suckered into increasingly using a system for finding partners that is inferior to the old-fashioned process of going out and meeting people? One might combine this with sexual liberation and casual sex - what happens if people increasingly refuse sex because they don't want casual sex and hate the idea of being another notch on some player's bedpost?
What if people just have other things to do? Maybe they'd rather concentrate on their studies/work, hobbies or whatever else floats their boat. Sex has become perhaps less important to them (which is of course bad news for people still really wanting it?)
How about pornography? The vast accessibility of pornography may make it easier for people to find sexual gratification in fantasy and masturbation than the real world - with the added possibility that pornography also gives them unrealistic expectations and ideas of what sex is that increase their anxiety when faced with the "real thing".
We could consider health issues. There's increasing anxiety and depression in young people - both states are known to depress sex drive, and the medications used to treat them also often do, as well. Perhaps despair and apathy is actually a wider social issue amongst the young relating to all sorts of factors (e.g. job prospects), which then impacts on sex drive. Rising obesity rates, both because of health-related potentially negative impacts of obesity on sex drive and function, combined also with potentially greater problems being found attractive? In a different form of health issue, concern over STDs.
* * *
Where I'm going with all this is that there a lot of potential reasons people may be having less sex. I suspect a lot of people heavily involved with "gender war" issues simply prefer to frame the issue in ways beneficial to their preconceived ideology.