Scientifically, how many times a week "should" couples be having sex?

Let's start here: "The Three Things You Can't Talk About" in most first-world, middle-class-ish existences are sex (which creates you), money (which drives a lot of people), and failure (which happens to everyone almost hourly, but no one seems to ever want to really talk about). This post is about No. 1: sex. If you Google virtually any variation of "…