From a strictly zoological point of view, the idea that long-term relationships between man and woman is totally unnatural is very uninformed. Some sort of 'marriage' didn't just spring up in all human cultures out of the blue, you know. That would be extremely unlikely. If long-term monogamous relationships were truly "totally unnatural", they would never have developed.
Natural=Produced by or present in nature.
Marriage is a man-made institution. Full-life monogamous relationships are rare in the animal kingdom overall. It is unnatural by definition. Just because it's "traditional" does not mean it is natural to marry. Marriage developed as a means for families to trade and share assets and to reduce the incidence of disease with children, who were dying regularly post-birth due to disease. Also, many marriages were arranged; basically a person with a high-social status would marry an individual with a low social-status, and that marriage was nothing more than a business relationship more than anything else. Marriage has virtually NOTHING to do with love in the original sense. You can love someone and choose not marry them.
Purely anthropological evidence for that is quite compelling:
- Human offspring are, by a very long shot, the most defenceless and slowly maturing offspring of any species. To have even a fair chance of surviving alone in the wild, a child would need to be about 7 before he leaves his mother, and even then, 7 years is probably too short for a child to learn but the mere basics of the knowledge needed in a hunter-gatherer society. Seven years is a huge time span for a species that probably lived barely up to 30 in the wild.
- Human females are, also by a very long shot, the most handicapped of all mammalian females during mid- to late pregnancy. That is because we walk upright, with the baby pressuring against the perineal opening, in sharp contrast to all other mammals. A mare or a bitch can run for it almost till parturition; a 7-month pregnant woman is in no shape to scurry up a tree or into a hole at a moments notice to avoid a predator. So she -and the survival of the species- needs prolonged support.
- Humans and bonobos (a subtype of chimp) are the only mammals that have sex all year around; other species have mating seasons, breasts are only perceived when they are to be used (lingerie manufacturers would be out in the cold in the same was true for women). But even bonobos have oestrus: they may have sex games all year around, but the females are only fertile at short and specific dates (lucky they). Again in sharp contrast, women ovulate roughly every four weeks, rain or shine.
Add two and two together: species survival dependent on long periods of special protection for the pregnant female + child needing being taken care of for ages + the man who stays long term to perform the protection mentioned is sex driven all the time + the female is soon again fertile after suckling the child = another baby coming; repeat cycle = a third one on the way..... (and with an infant mortality rate probably over 70%, that was very much mandatory for assuring survival).
It would seem as if long term liaisons between individuals of different sexes is an ethological condition of our species, more than a mere custom appearing out of thin air. Of course, culture did come in afterwards to enshrine life-long commitments and call them matrimony, but it was building on realities on the ground.
The only unnatural thing is to imagine that culture itself doesn't have biological roots. And that applies too to long lasting monogamy, of one form or another.
We often forget we've had the pill for about 50 years, the rubber for about 120, the industrial revolution for about 200, a really competent agriculture for a few millenia, writing for less than 5,000, bows and arrows for about 10,000... but we've been surviving barefoot and armed with a stick and a chunk of stone for many hundreds of thousands of years before all that.
Basically my South African friend said almost exactly what I was going to say, but I will add that, ultimately, none of this means that humans are equipped to be in exclusive, monogamous relationships. Yes, what you are saying is true, but none of it particularly means one has to be in a relationship. A woman doesn't have to be in a relationship to have sex, have a child, nor to take care of a child, now that doesn't mean it isn't an ideal circumstance, for certain reasons, but it's not necessary
. As for bonobos, you raise a good point there, since bonobos are the animal closest to humans biologically; there are many differences between most species and female humans in regards to female humans' menstrual cycle compared to other species' estrous cycles, but none of the reasons really prove that everlasting, exclusive relationships work at the end of the day, or are actually beneficial.