I've been thinking about the difference in the age at which men and women reach their sexual peak. Considering the difference, I wonder if leaving the option for non-monogamy open is in fact the most ethical choice?
Speaking in very general terms, men do peak at age 20 physically but probably mature in sexual technique between 25-30. Women are reported to peak somewhere between 30- 40 years old and continue on until menopause. Men experience a steady decline with some experiencing a slight increase in decline in the 40s. Add to this a typical 5-10 year marriage interval where this seems less threatening to the marriage, you have some lines on the chart crossing. Throw in children maturing, and you'll find the need and comfort level for either swinging or half open marriages starting in the early 30s and peaking in starting time at age 40. Wholly based upon many posts I've read on like mined forums and various articles.
One fascinating new development is the younger generation just arriving in their twenties now, about to toss this observation on it's head.