1. My first question about studies like this is always "how do they know this?". And I while I know I could find the study and dig into the setsils, I don't have to do that to know that this is the result of surveys taken over this time period. Unless technology develops to grant us a way to monitor and track the sex lives of people objectively and unobtrusively, that's just the best way can do. So any conclusions drawn really should be "the decline in people's surveyed frequency of sexual intercourse has gone down over time". Just to throw out some baseless speculation: could people in the past inflated their answers to appear "cool" or similar? Could there be cultural shifts pressuring respondents to deflate their numbers now? Personally, I'm inclined to believe the results of the study ARE true, but I'm not confident in that.

2. The decline of 3rd spaces, which is a big concept with multiple causes. Car-centric infrastructure, industrialization, women moving to the workforce, capitalism, technology, etc. It has become harder for people to have intimate personal interactions with others who live nearby. I believe the rise of things like social media, dating apps, and now AI companions is less about "hey we developed this new technology to replace and maybe be better than real human interaction" and more about "we need to develop something to replace what we have lost".

3. Consent. Reductions in arranged marriages and child marriages. Protections and rights for women and children.

4. Economics. Everyone is overworked and tired. I've seen this in a lot of the other comments here but I actually don't buy into this quite as much. There seems to be an inverse relationship between GDP per capita and birth rate, at least recently. Most of Europe, Japan, Australia, the US, Canada, Korea, and perhaps most notably... China. All have experienced declines in birthrates, and in a lot these cases there is good modern data showing the birth rates changing as these economies develop. The countries having the most children are poorer countries.

Now, it could be that these wealthier countries have access to birth control, so this does not necessarily dissolve economics as a factor. But, my own theory is that sex is one of the cheapest forms of entertainment available to humanity (if you don't factor in the costs of children). So the citizens of these wealthier countries are spending their time and money doing other things. Not just skii vacations or going yachting, but reading books and watching TV.