There's a fair argument that it's better than a fleeting non-hedonistic existence. And we can't do anything about the "fleeting" part, really, so we might as well enjoy the time we have. The birthrate is high enough where I live, and where most of us live. It could, if anything, afford to be a bit lower.
You caught my drift. Though I can't agree with anything related to antinatalism, even if only tangentially.
I am not sure most of the unmarried "herbivore" men in Japan would be classified as hedonistic by western standards. From my understanding (which well could be wrong) they can't balance the private life to work ratio, simply because they are expected to work a lot and put work performance above personal needs. That on top of a society which has a very intricate, complex and occasionally contradictory dating dynamic means that something's got to give. And since you need money to live but can survive without a family the choice is sadly simple, if disastrous for both the individual and society.
Afaik, that "herbivore" term means the guy has no ambition whatsoever, and thus it would be the easy lifestyle first and then whatever job that makes it possible. I wrote a bunch of alternative terms because they apply variably. Like over here and possibly in other Nordic countries being a welfare NEET for ever is a possibility. Though a closer resemblance could be to a semi-professional gamer/streamer who supports himself with a low-paying job.
If they are happy what's the problem? It's their life. If people want relationships and/or children they are free to pursue it.
Knocking on a topic that's bigger than this thread, but I'll summarize something off the top of my head.
Now of course maybe in Japan other societal factors are involved but I don't really think so. Marriage and fertility rates plummeted in the western world as well(particularly Europe, and then especially Germany). And Germany is probably one of the most luxurious welfare states with lenient working hours and child care compensations. I think the primary reason is that we don't have to conform to traditional role models anymore, and people are free to adopt alternative lifestyles. What probably also contributes is that people have gotten really ehm..critical in partner choice but that is most likely a separate discussion. We don't really 'need' each other anymore so it's mostly down to looks and physical attraction now.
Eventually society as a whole should accommodate to this trend of an aging population and many to most people being single. It also comes with a lot of positives like less of a need for constant economic growth that is wrecking the environment and stressing people out. The current economic model isn't sustainable anyway. So yeah if people don't have, or don't want, families and/or children I definitely don't think they should be stigmatized for that.
In the long run any antinatalism or "childfree" sentiments sort themselves out. However, during the course of a lifetime our way of life can doom itself, as dying societies have little authority in deciding the future. The liberal-minded people that consider maybe having one child think that the people moving in to keep demographics afloat will adopt their values instead of just their consumerism. I wouldn't bet on it.
About partners, there are still overwhelming trends in people's attitudes and what everyone
ends up with. Even the persistent singles usually get into a monogamous relationship in their 30s. (1-2% of men are of course "doomed" due to demographics, but that's one thing we can't do anything about.) But before that age range the average guy is at a massive disadvantage akin to the infamous 20/80 rule that online dating and big cities exacerbate further into the inflated expectations that you describe.