From Matt Ridley’s book “The Origins of Virtue” (1997: 87-88):
Imagine if sex were an activity normally carried out communally and publicly, but eating was something done secretly and privately. There is no particular reason why the world could not be organized that way, so that it seemed positively odd to want to have sex alone and rather shameful to be caught eating in public. No reason except human nature. It is simply part of our make-up that food is communal and sex is private. It is so deeply ingrained in the human mind that the reverse is unthinkably weird. The bizarre notion, beloved of various historians, that sexual privacy was a cultural invention of medieval Christendom, has long been exploded. All over the world, whatever god people worship, and however many or few clothes they wear in public, sex is a secret act to be done quietly when everybody else is asleep or out in the fields in the daytime where nobody can see. It is a universal human characteristic. Eating food, on the other hand, is just as universally a communal activity.”
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“The most fundamentally selfless and communitarian thing we do is to share food; it is the very basis of society. Sex we do not share; we are possessive, jealous and secretive, prone to murdering our sexual rivals and guarding our partners if given the chance. But food is something to share.”
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The anthropologist in me is hesitant to say almost anything is universal. Almost any pattern in human behavior has exceptions. Still, I read this book years ago and Ridley made me think about how strong these patterns held and how they came to be that way. Anthropologists?
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Patrick F. Clarkin, Ph.D. 
Just off the top of my head, food production and procurement is inherently social, and transparently so in face-to-face societies. Also, I think I read something David Graeber wrote about the way food has a way of blurring what is meant by ‘consumption’ that seems relevant.
To reiterate what Mateo has said, food has for as long as we know been the principle reason for organising social groups. Having produced in commune, food is shared in common. Reproduction on the other hand, since only one biological father is possible per infant (unlike my dogs) involves only two people as the norm. Sounds pretty straightforward and I think it is. Why viewing the act is taboo is another question. Taboos are taboos are taboos. Asking why is itself taboo.
To Robert, and Mateo, this makes sense. If production of food was social, it makes sense that its consumption should be too. I wonder though about the universal claim. We can find busy urban people eating alone in fast food restaurants, or in offices at their desks.
Yeah, but are they an alternative or an aberration? There are also people who make their living having sex a dozen people at a time for the world to see. (Which is not to say that pornography is as devoid of joy and debasing as subsisting off of fast food.)
With sex, hormones are release that create or strengthen a bond. I suppose being public with sex would interfere with the oxytocin and prolactin release.
Hmm. I’m not sure about that, Polly. It’s testable, though.
Excellent reply Patrick — but don’t get arrested while testing Polly’s hypothesis.
As to the busy urbanite (or the hermit monk) eating alone — he is part of society too, but unlikely to be producing food. A lone person in a fast food restaurant is eating communally — simply the visual complex of the commune in this case is not relatives and friends sitting together to eat. The society of strangers thing.
But sex alone is a bit sad — I suppose that is why we have the internet.
It has always intrigued me why intercourse or masturbation in public can land you in jail, and if viewed by children is considered one of the most vile crimes in the book, while gorging on barbecue ribs, ice cream and chocolate cake is not only accepted but openly glamorized conversation, as well as in television and movies.
People go so far as to assume that seeing sexual behavior is harmful and traumatizing (especially to children) while openly engaging in gluttony multiple times daily, without batting an eye.
Curious blog post, and interesting thoughts.