- Sucked into virtual worlds. Teens spend less time physically with others.
- Pressure to study.
- More structured time. Less time free.
- Less physically attractive kids due to rising obesity.
- Lower male sex drive due to rising obesity's effect of lowering testosterone.
The latter two also seem true, but while that may be a limiting factor on the boys' side of the equation, even at a young age girls have primarily control access, and chubbier girls put out more easily and often (the GSS reveals the same). So I'd expect to controlling access to be moving a bit closer to parity (though with major dimorphic differences still remaining), but wouldn't assume the net effect is less overall sexual desire.
This strikes me as yet more empirical evidence refuting the Game narrative that in today's man-hating world, women are looser than they've ever been before. Time and time again, the data contradict that narrative. It's worth considering others that are more empirically grounded.
As someone who actually spends a significant amount of time with girls in their late teens and early twenties due to personal preference, this meshes with my own personal experiences. It always takes me several months of contact to really get intimate with a hot girl at that age. Part of it may be the inherent caution in getting involved with someone nearly ten years older than themselves, but even reflecting back on when I was a teenager, it feels like there is more resistance now than there was then (and my game has definitely improved since then!).