This is one of the many ways old models of marriage are breaking down. Lifetime contracts of monogamous exclusivity sound like promises of happily-ever-after, which of course belongs in fairy tales.

I'm not sure about your premise that jealousy is endemic to human nature. It is definitely our nature to be anxious about changes. And it is typical of us--though we can learn better ways to handle it--to respond to anxiety with efforts to control, to resist the change.

I like your idea of "hall passes." But what I think would better address this and many other problems is to expand our idea of what a marriage should be. A family of 4 to 8 adults plus all their children would offer greater financial and emotional stability, and a problem in one person or dyad wouldn't threaten the whole system.