What they call (incorrectly) 'sex' is not mating.
Having kids is strongly inversely correlated with 'having sex'. (People who are most successful at reproducing are also the people with the fewest sexual partners and a less active sex life.)
Having kids is strongly inversely correlated with 'having sex'. (People who are most successful at reproducing are also the people with the fewest sexual partners and a less active sex life.)
My interpretation: this is largely because most people, when they are single, struggle to find romantic partners. Many are also (at least ostensibly) looking for someone to marry, and so they try to optimize for that (sometimes) at the expense of more sex now.
I agree with the distinction you are making though.
Moreover, some of the most successfully reproducing people are those who have sex once a year.
'Sex' is, by definition, something that's supposed to lead to reproduction. Whatever you're discussing here isn't.