This has a simple explanation that the article doesn't account for, namely that the busier you are with work at a young age (and hence better income), the less time you have for sexual activity within or without an associated relationship.
The thing of interest to me, though, was the gender-based age difference in the inversion point of the money/sex correlation. I would have expected the opposite, frankly, given society's "breadwinner" v. "trophy wife" archetypes.
It doesn't surprise me that females aged 20-30 don't show this spread.
I'd expect to see a similar dynamic in sex versus grades for college age males. It's the stereotypical example of partying versus studying on a Thursday or Friday night, with $$$ substituted for grades!