Modern casual sexual encounters are assessed purely on subjective ratings of the opposite party's sexual attractiveness (secondary sexual characteristics, behavior, social status, etc.). Furthermore, constant saturation into the full range of these behaviors and characteristics across humanity through the internet and dating applications creates a endless "could I get a better mate?" feedback loop in the minds of both men and women. You're no longer competing within a social group, you're competing within every social group.
Notice that the per the article, the population of "sexless" men has grown nearly three times as much as the population of sexless women. Fewer men are having more sex; it's practically on-demand for them, and they are usually tremendous specimens when compared to other men.
If you are not aware of this, I would bet that you do not know any men who are on the "winning" side of the new dating economy. If you do know men like this and you disagree with me, please elaborate.
I have heard a great deal of insight into "the dating market" ranging from the perspective of attractive athletes (both male and female) to the perspective of unattractive nerds (both male and female) and this pattern is clearly visible through their stories. The women tend to retreat from casual sex because they don't want it (and not only because the pickings are subjectively slim, but also because the expectations for them to 'put out' are very high); the men tend to retreat from casual sex because they can't have it.