2. The evolutionary instinct to have a “back-up plan.”
The only place there is some evidence of a female sexual peak later in life is that there is a spike of female infidelity in the late thirties and forties, particularly as a woman reaches the end of her reproductive years. Evolutionary researchers have suggested that this might reflect women pursuing a “backup plan,” or another man to care for them and with whom they can have children if their current mate abandons them.
I don’t know about this argument — for instance, in my book, I talk about a Roman tradition where men who had had multiple children with their wives sometimes loaned her to other childless men, so that she could bear them children. I’m just not sure that in humans the male desire to obsessively and jealously procreate is as strong as some theorists suggest. I also believe these same women are likely reaching a point of economic independence from their own careers and finances — a factor that strongly predicts female infidelity.