Patrick Boyle has looked at causes of decline in the total fertility rate around the world and concluded reduction in the formation of couples (married or not) is a major cause.
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https://www.ggd.world/p/the-global-collapse-of-coupling-and
I wonder if the ultimate cost (in lives never born) of technologies like smartphones and Tinder will be. If a significant part of the entire youth cohort never learns to interact with the other sex in the real world, we might be looking at a pandemics of loneliness - and at subsequent global birth deficits in eight to nine figures.
If coupling is reduced, it should become the norm for women to have and raise children alone, or at least without a male partner. This increases the burden on women, so at the same time the number of children they must support would have to decrease.
This could be achieved technologically, by filtering sperm to remove Y chromosome carrying gametes. As a result, the female/male ratio of newborns would dramatically increase. At the US TFR of 1.66, a 2:1 female to male ratio would be more than enough to maintain the population. Even higher ratios could be imagined, leading to an almost entirely female population.
Social engineering to reach this state is left as an exercise to the science fiction writer.
The research to do this seems to be close to succeeding - see e.g. https://www.newscientist.com/article/dn11601-bone-stem-cells...
When researchers perfect this technique for humans, which they almost certainly will eventually, there will be no need for men at all. Women will be able to obtain lab-grown sperm from other women, and use this for fertilization.
All of these female-originating sperms will be X-chromosomed, and so in the much longer term we will see the eradication of the Y chromosome - and all the problems that arise from it.