Which is the more "natural" human relationship?

Which is the more “natural” human relationship? In a threesome, Man-Woman-Man or Woman-Man-Woman?

Throughout history, more societies have had the practice of men taking multiple wives than the practice of women taking multiple husbands. That’s probably more cultural than natural, though, although Jared Diamond in his book “The Third Chimpanzee” says that human genital size compared to the other apes might lead one to suggest that humans are naturally mildly polygynous.

From a biological standpoint, a polygynous relationship would seem to lead to more kids than a polyandrous one, because a man is able to get multiple women pregnant at once, while, no matter how many husbands a woman has, she can’t reproduce more than once every 9 months.

However, that being said, I think neither is more natural…I think the familial relationships we set up are much more cultural than biological, and set up to fit cultural needs. So, from that standpoint, monogamy, polygyny and polyandry are all as natural, or as unnatural as one another.

If it happened throughout history then I would consider it more than cultural, and possibly more ‘natural’. Though, really, I would only call man-woman ‘natural’. And why would human’s genital size imply anything about number of sex partners?

I think that if polygamy were natural, we would have at least twice as many women than men, or men than women.

It is more about testicle size and amount of semen produced than genital size per se.

Male chimps in a tribe tend to all take a turn at a female in estrus. They produce more semen per ejaculation than a human male, presumably to flush out the sperm or rivals. Gorillas produce less semen, and (I believe) only the dominant male mates with receptive females. They do not need to compete in the fallopian tubes with rival sperm.

Male chimp genitals are larger in proportion to their body size than male human’s, and humans are better endowed than gorillas. The relationship between semen/sperm production and mating strategies track fairly well across all mammal species. I remember some whale research, but I cannot seem to dig it up right now.

That makes absolutely no sense whatsoever. The gender of a child is pretty much a 50/50 tossup regardless of how many partners each parent has. Sex could be a wild orgy with couplings taking place indiscriminately but among mammals, biology still defines actual reproduction as being the mingling of genetic material from one male and one female, with XX and XY results being about equally likely.

Gender imbalances can and do occur but typically with two causes:
[ul][li]Female infanticide and/or selective abortion[/li][li]Sustained warfare, with male soldiers being killed in large numbers[/ul][/li]
This doesn’t count the imbalance among the elderly, mind you, which is due to females having a longer average lifespan. In any case, our primate forebears stumbled across a formula that meant enough females were around to produce sufficient offspring to sustain the tribe’s numbers, and enough males were around to fight off attackers and protect the tribe’s territory. Polygamy is completely irrelevant.