Recently, an acquaintance told me about her brother’s marriage break-up. “They had sex once in eight years,” she said – as if that were the sole reason. It would not have been: the couple’s collapsing relationship was the reason. The absence of sex – except for that single occasion in eight years – was a symptom that likely made it worse. It raises the question, though: is there a sweet spot for sex in a relationship? How many times a week or a month should we be “doing it” to keep a...