I definitely do focus on economics as the main reason why people aren't having children in the numbers necessary for a reproductively successful species, and I stand by that analysis which I haven't finished.  Working on Part 2 most of today while I waited to see if anyone would chip in.  Why  they're not having children isn't the same reason why they aren't as interested in having sex though, at least in the industrialized societies.

You mention the survey of Koreans who have not had sex for the past year, 43% of the women and 29% of the men.  In the course of my reading for this, I turned up a quote from another survey which said 50% of Japanese women report they don't even like having sex.  As you point out, since women now have the ability to be economically independent without marriage, they don't really have to have sex if they don't like it.

Where this comes into play is in courtship and then marriage.  Although not necessary for having children, marriage generally makes raising them a lot easier.  Being a single parent is very difficult even beyond the economic aspect.  Children who have both a male and female role model around tend to grow up healthier and more balanced, regardless which sex they are.  Coping with problems the child may have growing up is easier if you have someone to share the burden with.  So, decreasing marriage rates will clearly lead to decreasing TFR.  If the 50% of women who don't like sex choose to stay single and not marry, that's going to radically draw down the average number of births per woman in the society.  I think if you look at all of these Asian cultures, you will find decreasing marriage rates to go along with the lowered birth rate.

While this cultural shift seems most pronounced in the Asian countries, it's definitely also true here in the FSoA, based on articles I have read about teenager's attitudes towards sex.  Sex does not seem to be as important to them as their smart phones.  Physical interaction in general isn't as valued, particularly after COVID.

While sex with the opposite sex seems to be on the decrease, sex between members of the same sex appears to be on the increase, at least if you go by what you see in the media now.  Whereas when I was a teenager it was a huge controversy if a character in a sitcom was gay, now if a sitcom does not feature an LGBTQ couple it's cause for media outrage.  Wth the Korean women 43% of whom said they haven't had sex for a year, I wonder if that includes sex with other women?  Same question would apply with the men.

Also not addressed are how many of these folks are having a lot of sex, just they have it by themselves.  My bet is that masturbation is at an all time high, and certainly with the Japanese pornography has been turned into a high art form.  The bizarre fantasies of Japanese pornography can't possibly be equaled by any real life sexual experiences, and the selection of sex toys available for sale on TEMU is literally out of this world.

I think the dislike of sex is a consequence of overcrowding in the urban environment.  Being at close proximity to so many people all day, when you go home at night there is nothing you want more than to be alone, not squished up against another person in bed all night.  The rows of people on phones in call centers or on production lines, squishing onto the subway like sardines morning and night, you have had more than enough smell of the sweat and pheromones of other homo saps by the end of the day.

To conclude, I agree with you that a decreasing desire for sex with other people and a reduction in coupling up and marriage definitely contributes to the lower TFR.  It's still a consequence of the economics though and the urban style of living promoted by industrialization.  Living inside the city is where you find most of the bennies of this type of civilization.  The clubs, concerts, theater, fine restaurants, big sporting events, all at your fingertips.  All this stimulation provides a substitute for sex, which is about the only exciting thing a poor farmer has to look forward to doing at the end of the day.  Thus it is the poorest countries still making all the babies.

RE