My hypothesis is the decline is caused by a multifaceted change in environment:

• cost of living crisis caused by inflation, folks are making only about 1/3rd what their grandparents made for the same labor
• debt crisis caused by the above, crippling debt causes anxiety
• educational treadmill, folks are working harder for top jobs, so schooling takes longer and is much harder
• hormonal crisis, many young women are on hormonal birth control which is well known to significantly reduce libido in a large percentage of the population
• food supply crisis, cheap, poor quality highly-processed food make folks just slightly less healthy all the time, possibly can affect libido with lowered testosterone caused by preservatives
• social media crisis, evolution caused us to want to be nosy about 70 members of our tribe, not hundreds of millions. Social media is causing us to have the social equivalent of Daredevil's acute hearing, but it's addicting so folks can't easily shut it off (as I write on social media...)
• fitness crisis, many folks just aren't healthy enough to lift and run, so they have less testosterone which lowers libido for everyone regardless of sex/gender
• reproductive fear, folks realize they have it bad with little hope of improvement, so they don't want the same for their offspring
• perfectionism causing a lack of confidence, being able to see the best in the world at any second on your phone causes acute perfectionism that prevents folks from even trying. Look up rates of musical instrument playing to see how few are even starting compared to 20 years ago. Skills in an area lead to increased confidence which makes a person more confident
• protective parenting combined with extremely restrictive schooling, leads to less "free time" where folks can hang out and build social skills
• hyper niche porn on demand, every person can access the porn of their choice on demand at all times. This causes unrealistic tastes and lack of confidence.
• infection fears, everyone is taught from a young age to be afraid of STDs and pregnancy, so it's a looming fear in many
• entertainment quality, we've created the most addicting games, movies, shows, and apps. These make it more interesting to sit and zone out than socialize
• anxiety/depression crisis from the above leads to a population with lower libido, SSRIs are well known to reduce libido in a large subset of the population

All in all, we've got a society that is nearly perfectly designed to reduce libido in the population. Those that manage to avoid or ignore these challenges find a very small dating pool compared to decades past. Those that pair off are less likely to have offspring, such that we will likely have a population collapse in the next hundred years. Hopefully by then have figured enough of this out that we build a society that makes it easy to be a human who finds joy and love in other humans and not in a screen.