I haven't made my way though the entirety of the topic yet (I'm working on it), but there's something I wanted to contribute before offering anything about anything.

Treason, a debate over such a thing I think is a fine idea, but your assertion about opinions is absolutely incorrect. An opinion is typically held due to the values held by the person with the opinion. Looking at the base of the opinion, the value itself, it is impossible to empirically prove or disprove the value, or claim the supremacy of one over the other with a factual basis, as 1) such a thing can only be examined in relation to other values, which excludes in large part any tangible evidence (in this case, the only evidence that could be provided would be a comparison between testimonials from monogamous and couples like your propose, but that speaks more to an examination of monogamy than a value) and 2) the argument necessarily introduces bias from both sides in regard to each side's arguments. As such, one's view on the worth of monogamy being based on values, something which cannot be examined in a rational, scientific manner, leaves the matter of supporting an opinion such as this impossible to support, thus leaving the only validation the opinion has lying in the values of the individual, something which is impossible to empirically validate. Thus, I argue entitlement to one's opinion is a valid notion in this matter.

I would, perhaps, agree if the opinion was on something tangible, such as, say, whether product A or product B is better at doing job C, as that can be supported with evidence. Just a caveat.

My point in this matter is that if you're looking at the underlying reasons that prompt monogamy or not, you're looking for evidence that does not exist, that cannot exist. If you're looking purely at monogamy vs alternatives without any consideration as to why the belief is held (which would invalidate, according to your topic title, your point in bringing this up), then you'd be more reasonable in expecting evidence.

Now, I'll get back to reading the topic...saw the "opinions" line and had to weigh in.

Harley