I blame smart phones, but blame is a strong word. We no communicate with texting, social networking, and many other means than in-person conversations. Teenagers are electing to get a drivers license later in life because they don't need a car to be connected to their friends. Less in-person interaction results in less opportunity for intimate interaction.
I blame the womens movement, and that's a good thing. More women are placing their careers first, and getting around to having a family later and later in their lives.
I also blame the economy. The recession ended technically, meaning we stopped getting worse. But our economy isn't seeing the wage growth that would indicate a prosperous environment that encourages recreational activities, and procreation.