It’s not like there are 1.5 billion Indians because no one has sex there.
Both are true in large part there, at least in the cities.
Except Indian politics are at least 10x as crazy as current US politics on the ground, and probably 10x as potentially (violently) serious if someone ‘steps out of line’, so people are better at hiding what is going on.
The amount of sex though definitely includes hookups and a lot of sex outside of the acknowledged marital structure. A large number of those kids may well be illegitimate, I suspect no one wants to look too hard.
I also suspect what is happening in the US is a combination of defacto ‘strikes’ from both sides of the equation, combined with general confusion as to what to do or why. Essentially a ‘why would I want to engage with this mess? What’s even in it for me?’.
(Not sure if there are directly comparable surveys, but I wouldn't be surprised to discover that unmarried Americans were rather more sexually active than unmarried Indians, even with the downward trend)
Good luck doing that in India without being murdered by your parents (not joking!).
Happy to be shown otherwise if there's data?