For years, women have been trading sex for investments. Investment in themselves, in their children, in their family, in their security. For years, women have been doing this, and men have been willing to pay out. Apparently, it's a mutual thing. I'm not sure why this surprises anyone, since we do see that kind of behavior in other animals-- think elaborate nest building by males of bird species. Think territory and protection for females of predator species.
Why do we think we're really any different?
Humanity stumbles on this roadblock EVERY time. We think we're special. We don't understand that so much of what we do, how we live, is wired into our makeup. Maybe we're so smart that we don't recognize instinct when it takes over. We're too good at rationalizing when we do things we don't understand. But we're hard wired, just like the rest of the animal kingdom. Why should it be a surprise that we're also hardwired when it comes to why we have sex? Lets see--there's all the selfish reasons, personal survival and making life better... and then there's the big one. The one that encompasses everything, because it allows us to EXIST.
Getting something out of the deal isn't just for penguins anymore.