Patio, you said;
In the book, "The Third Chimpanzee" Jared Diamond makes the point that based on animal studies and studies of the history of humans, that we are MOSTLY monogamous. As are a lot of other creatures.
Mmmm... I'd need a LOT more detail before I agree with the 'As are a lot of other creatures' bit. Modern genetics allow scientists to examine the genetic make up of offspring. Even in species previously thought of as monogamous (many birds for example) they are finding that the rate of 'adultery' is very high. None of our closest relatives practise monogamy. In human populations I've seen figures of well over 16% for babies NOT fathered by their mothers' partner.
I think humans are capable of sexual monogamy. It might not be natural, but culturally we can certainly be raised to expect and strive for monogamy.
I think that guys are a little worse at monogamy than women.
As for gay partnerships... well, I've known and seen very long term committed gay couples, but that's not enough for me to form a reliable opinion.
I do think you can be emotionally monogamous, but very few girlfriends have bought that line, as as far as they are concerned I am tupping some other girl and they don't care if I love them more.
Basically, I see it as part of my basic understanding of humans as CFM's (Clever F*cking Monkies).
Humans have evolved the partnership patterns they have as a result of the biological changes our species have undergone.
If we look at the first stages of attachment, they tends to last about three to four years. Across many cultures the rate of divorce, where this is allowed and or acceptable, peaks around three to four years.
This period of time matches the period of time where our big-headed (proportional to body size), helpless, slow growing babies are actually old enough to fend for themselves and be mobile to some extent.
In hunter-gatherer societies a four year old child can gather over 30% of it's own food. A four year old can also walk reasonable distances, especially one that has grown up in a rough environment where it is the sole method of transport.
This means that we are instinctively programmed to be susceptable to a pair bond that tend to last the minimum amount of time to get a child to an age where the mother could move on with the child if the pair-bond was 'not working'.
At this stage, the pair can obviously stay together. But the bond that kept them together when it was vital for their offspring to have two parents changes.
Of course, a female within a pair-bond can still explore genetic diversity by having sex with another male, while still benefiting from the security of the pair-bond she is in. Likewise, a male can spread his gene juice around, and still have a clean bear skin rug to come back to. So to call these pair-bonds 'monogamous' would be a bit of a misnomer.
That is a very condensed version of what I've read on the subject, so apologies if I have mistated the theory through failure of memory - it's been three years since I read everything I could get on sexual behaviour in Homo sapiens, so I'm a little rusty.
The fact that human children are high maintenance means that it is unlikely our early ancestors would of had polygamous groups, with one male and many females or vica-versa, until they developed for societal reasons. Likewise, because of the high maintenance thing, it's unlikely that our ancestors would have mirrored chimps or Benobo, where simplistic descriptors like 'polygamous' and 'mongamous' don't really fit.
Of course, we have our Western Christ-Judaic cultural imprinting to deal with here. We might not be having a conversation asking if monogamy is normal (to paraphrase) if we came from another culture.
In some African cultures there are 'marriages', but it is quite possible to have 'stylised clandestine' (i.e. everyone knows they go on, and you could catch someone at it easily if you tried, but everyone ignores it politely) sexual liasons for both men and women.
These clandestine relationships very rarely damage or supplant the marriages as they are an accepted part of life. Similar cultural differences can be found in different societies attitudes' towards intra-adolescent relationships, which can differ far from our own.