A recent Gallup poll which resulted in near unanimous disapproval of cheating also found rising acceptance of many other non-traditional, consensual sexual relationships. The new ethical consensus that you can do whatever you like as long as you are not hurting anyone — and as long as you’re being rigorously candid— reflects a thoroughly modern mix of tolerance and puritanical censoriousness. We’ve become more willing to embrace diverse models of sexual self-expression even as we’ve become ever more intolerant of hypocrisy and the human frailty which makes hypocrisy almost inevitable.
A Google search for “ethical non-monogamy” yields plenty of results about polyamory and open marriage. It also brings up sex columnist Dan Savage’s notion of “monogamish" relationships, in which partners pledge enduring emotional commitment while enjoying "flings, affairs, three-ways, and swinging experiences."