Robin Rinaldi’s marriage was not a sexless one. “When I polled my married friends,” she writes, “I realized how exceptional was the fact that after sixteen years we still had regular sex once or twice a week — sex that lasted forty-five minutes and often ended in joyful tears.” Despite this exceptional sex life, though, she felt at the age of 44 that something was missing — maybe it was a lack of passion, maybe it was that she had slept with only a few men before marriage or maybe it was that her husband had squashed her hopes of having a child by getting a vasectomy. In her new book, “The Wild Oats Project: One Woman’s Midlife Quest for Passion at Any Cost,” she writes, ”I refuse to go to my grave with no children and only four lovers. If I can’t have one, I must have the other.”