***SPOILER*** Today, talk of performance-related erectile dysfunction is on every woman's lips, if you'll pardon the expression. Group or open sexuality, for the uninitiated straight first-time "vanilla" male, particularly in the same room/bed with another male, can be a very stressful situation.
Simply put: Despite the appeal and willingness of Carol & Alice, neither Bob or Ted, in their situational anxiety, were able to "get it up". Watch carefully and you'll see the disappointment on the faces of the women.
As the former public relations director and spokesman for the 1970s Sandstone Retreat (imdb: "Sandstone") I often compared the psychological benefits of well-introduced group sex with the well-guided initial psychedelic experience. Both experiences often result in highly euphoric, life changing, long lasting insight.
Finally, B&C&T&A, despite the wardrobe, is by no means a quaint relic of the swinging 60s/70s. Real life realizations of their entirely rational human impulses occur every day and night in every large city and small town around the world.
The Sexual Revolution, and the realities of polyamory, polyfidelity, the swinging lifestyle and safer sex practices remain alive and well in God-fearing America in the 21st Century.