It is easy to find post hoc "explanations" for a finding you did not expect in the first place. That is not science. Science is about testing theories. No theory about the decline in sexual frequency has yet been tested. It is a puzzle. This is mine https://plus.google.com/101046916407340625977/posts/8o3isk7Nc1g …
I don't see a conflict. The theory says, with a higher sex ratio (more men), women have greater power pursuing female sexual strategies, which means a preference of quality over quantity. The sexual revolution in the 60s was the opposite.