If anything monogamy is unnatural. If being moral equate to being natural, then... That would have a lot of unacceptable consequences.

Monogamy is as natural as polygamy.

Point taken. Good one at that.

:wink:

I always eat my partner after sex. It's only natural.

True but possibly immoral.

Being trustworthy is a virtue. Monogamy involves trust.

If arranged by contract, either 'marriage' should be kept according to the understanding at the time. If a person cheats, and the process was intended to generate a monogamous relationship, cheating breaches the contract.
It's just me, but I feel that, based on MY arrangement with my only 'partner' in marriage, an open marriage is an oxymoron. Even if both partners agree to 'stray', or to seek hedonism with others, if their original contract was entered into under the rubric of a 'church' wedding, they are both in breach. They should both agree to end the marriage and then formalize it as something else...or not.

All contracts are meant to build formal trust and predictability. Any breach is theft because it robs the injured party, or if you prefer, the one still feeling bound by the original terms, of that trust and predictability. It is as injurious to a marriage as it is to society as a whole, and for the same reason; you can't build a society in chaos, deceit, theft, self-indulgence, and lack of compassion or empathy. In short, you have no relationship if it provides no avenue for trust and for predicting the maintenance of the terms originally expressed and promised by the several parties.

I recall one of the Seven Duties of William Ross. It is 'promise keeping'.

However, if several people agree to form some form of relationship among themselves, at least it is legal and done with informed consent. Even so, if one of them strays outside, they are still in breach.

Black widow spiders and praying mantis agree, so it is only natural.