I am not trying to argue that monogamy is good, but why it came about and for what reasons.
Sorry, I guess I read some implications in there that you didn't intend.
Also, historically divorce rates were at 0%. Does this prove that monogamy is good? No, it is just a single unrelated fact.
It's not unrelated, but it is debatable how it affects things. My contention is that divorce rates were 0% (or very low, after that) because women in previous centuries and generations were second-class citizens and couldn't leave bad marriages (and the same also went for men, but to a lesser degree; society frowned so much on divorce that it just wasn't done). The rates are much higher now because women have equal rights, and are able to have their own careers, so they don't need men to be meal tickets any more, so people don't stay in unhappy marriages like they used to. All this points to the idea that monogamy and life-long marriages are simply a bad and unworkable idea for most people. In fact, in centuries past, love wasn't even a factor in marriages, only convenience and politics.
the proven method of living like animals in small tribal units.
The problem here is this doesn't work so well with modern society. However, there are more and more people joining up into polyamorous groupings, which do resemble tribes, and have as one component resource-sharing. I think we'll see a lot more of this in the future. It's not at all unlike Robert Heinlein's "line marriages". In generations past, people used to rely on their extended families a lot. These days, people are more mobile, and also frequently don't really like their extended families, but with polyamorous groupings, people only associate out of freewill and interest, not because of blood relations.
But beyond that, I would argue that the family unit incentives and protects the post-fertile woman more than the tribal sharing society of old did.
I disagree. Some people are luckier than others and have better or bigger families. I know lots of people whose families don't give a shit about them. Tribes don't have this problem so much.
The problem with old tribal societies, of course, is that they don't really work in larger societies that were enabled by the development of agriculture.
You seem to be rather refuting your own arguments here. Was agriculture capable of providing more food or was it less food?
I think it depends on what exactly you're comparing. If you compare early agriculture to hunter-gatherer societies in their peak, it's probably less. Think about it: why would you expend so much effort sowing seeds and tilling dirt when you can just run around and pick plentiful naturally-growing stuff? The problem is that, as human populations grew, there wasn't enough naturally-growing food (flora or fauna) to support those populations, so people invented agriculture. Modern agriculture, of course, can provide enormous amounts of food.
The other problem is that agriculture doesn't provide a very good diversity of food; that's why people lost a foot of height when they switched (there's archaeological studies about this). These days, however, we've made up for it thanks to large-scale trade and transportation, so obviously a modern grocery store has an enormous variety of foods from all over the world. But in 2000BCE this wasn't the case, and in fact it's only been recently that people have gotten tall again.
I would argue that agriculture was the worst thing to ever happened to humans and the entire planet but that is mostly a personal preference not a fact.
That's definitely personal preference. Today's large societies are also why we have computers, the internet, smartphones, space travel, etc. Small societies simply cannot develop these technologies, nor can they develop medical technologies and knowledge which allow people to routinely live to 100 and not die of common infections.
I do agree we have a population problem at present, but it's because we don't use our resources well and we don't manage ourselves well; we still have to coexist with our natural environment, and we're screwing that up badly. We should be able to develop the knowledge and technology to coexist better with the environment, and later perhaps build our own habitats (including in space) instead of taxing the natural one here so badly, but for now we're acting like it's 1500 even though our population is far, far larger and our technologies far more polluting.