I guess I'll have to take your word on it, as I don't know nor do I care to know the slightest bit on how straight mens' minds work (assuming such a thing exists
[OBLIGATORY JOKE]).
Heh. It helps having a good background in behavioral biology.
That said, the very fact that polyandrous societies exist is proof that evolutionary fitness is not an override to a desired relationship status. If that were true, complete homosexuality wouldn't exist.
You dont seem to understand. In the few polyandrous societies that have existed, one female typically marries siblings (in most cases I am aware of) and the sex ratio is already so male biased that the polyandry is the only way the society can work at all. This happens in cultures with selective culling of females more often than not.
In cases like that, polyandry is the best way to maximize fitness, and problems with cuckoldry are mediated by the other male partner being a close relative.
Homosexuality is something completely different. It is the result of one of (or a combination) several possible causes. Genetic byproduct of a necessary quantitative trait locus (essentially, if you want brain development to work, a certain percent end up gay or trans thanks to the way genes recombine with sexual reproduction. This is fixed and selection has a hard time getting rid of it), Epigenetic maternal effects (fraternal birth order effects), Epigenetic parental effects as the result of Sexually Antagonistic Zygotic Drive (theoretical concept that has the potential to explain a LOT, including homosexuality)
Yeah... not to digress, but completely different is the point.
For one, you are talking of obligate polyandry, while I am talking of potential polyandry or polygamy. The two types of culture are completely different- one of them, to survive or prosper you need multiple spouses. This culture, it's merely an issue of "people who want to marry other people should be allowed to".
It does not matter whether it is obligate or facultative/ optional. Even if a very small portion of the population engaged in polygamy (and it will probably be nearly exclusively polygyny) it can still drastically alter the operational sex ratio.
In this one, men who want more than one wife would have more than one wife. They wouldn't be mandated to. I doubt very many would, especially if sex and marriage stop being associated. Women who want more than one husband would have more than one husband. I doubt very many would, especially if marriage stops being a means to gain children.
Except that they would not be in equal proportions.
Look, a male WANTS more than one exclusive mate (read: exclusive to him) almost every male does. It does not matter what culture he is in. The saving grace is that unless there is differential power between males such that one group can enforce their will on others, the society will not stand for it. Why? Because of social problems caused by it reducing their competitive ability (classic case of multilevel selection). Cultural evolution can and historically has strongly discouraged this in more egalitarian societies. Instead males try to have their cake and eat it too by having multiple mates, usually one exclusive and then one or a few on the side (read: they cheat)
If polygamy is allowed, those with more wealth will be able to attract and support multiple females. Females accept this because it is in their best interests to divide the attention and parental investment of one extremely wealthy male between them, than for them to try their luck with a lower status/wealth male.
(there is a reason Hugh Hefner has a harem. It is not because he is good looking. It is because he is wealthy and of high social status)
If only the Top 10% of males (in terms of income) engaged in polygamy here is how the sex ratio would be biased.
Top 1%-5 wives
Next 2%-4 wives
Next 3%-3 wives
Next 4%-2 wives
We will assume a male population size of 100, and a female population size of 100 for a 1:1 actual sex ratio.
10 people have 30 wives. Which leaves 70 wives for 90 people. This leaves 20% of the male population un-mated or having to rely on cuckoldry or rape to spread their genes. In order to obtain access to females and avoid being in that lowest 20% of the male population males will be forced to increase their social status. To do this will require taking additional risks over and above what they will normally. With each subsequent generation, because (if we assume a stable population size for the sake of simplicity, it does not affect the conceptual basis for the model) the lowest performing of these males gets near zero fitness. This sets up a run-away selection model, selecting for males who engage in more and more risk (eventually more crime to obtain wealthy or display desirable masculine traits), or who engage in more and more rape and cuckoldry in order to increase their fitness.
This is why western societies do not allow polygyny. The real reason anyway. It causes problems like this, that need to be compensated for, and the only ways to do this are extremely costly.
How common is group marriage? It's got to be vanishingly small among western culture (because there's no need for it, so it's only if by some strange chance three people end up loving eachother). The chances of finding someone whom you want to spend your life with squared, worse since a double coincidence of wants must arise in all of the people (or even a triple coincidence of wants, for Δ formations).
See above.
That said, "It'll cause societal ill" doesn't strike me as the best argument against something that'll happen anyhow just with absolutely 0 regulation and documentation. Especially not if it's on the order of a thousand single men caused by this.
Without marriage, the situation is fluid. People can move from one polyamorous relationship to another, drop out of them etc. Marriages are more binding.