It's late, I'm not going to look this up, but I think that the ~100 gm calculation comes down to nutrient balances.

Your "average" adult, excretes a certain amount of nitrogen per day... the breakdown products of cellular metabolism. To replace that, a source of nitrogen is needed. Protein is the richest source of N in the omnivores diet. To provide all the nitrogen requirements while maintaining the requisite balance of essential amino acids (ie those we can't make) a minimum of just over 100 gm of high quality protein is required. Eggs are a good source. But then you miss out on those good fish oils. And every diet needs just a little fat.

NB These calculations are for adults. Growing children are a different story.

Like anything, over consumption bad, under consumption bad. A little bit of everything.

The book, Omnivores Dilemma is a reasonable read. PETA probably hates it.

Okay everyone a reality check.

Ruminant animals (like cows) have multiple stomachs that act as fermenters to digest cellulosic plant material. They evolved as grazing animals that can digest and get energy out of material that most animals (mono gastric, one stomach animals) can't use. Modern feed lots have substituted grains (mostly corn) to improve weight gain and feed efficiency for profit reasons in beef and milk herds. It makes meat and milk faster, and until recently, cheaper than allowing cows to graze grass the way their physiology was designed to do.

I don't disagree that most westerners could use less meat in our diet. That misses the point however that meat has been historically cheap because grain has been historically cheap. This has produced an over supply of meat in western countries (keeping the price low) that is produced in a non sustainable way. Putting more grazing animals back on pasture lands rather than, say growing pasture land for cellulosic ethanol, could help moderate the rise in meat prices and encourage farmers to be good stewards to the land.

There are many studies out of the University of Wisconsin that high intensity rotational grazing practices are both good for the animals and good for the soil. But these approaches are labor intensive and only scale so far. They are not competitive with enormous feed lots when grain prices are low.

So my point is don't try to take the animal out of the equation but change the farming equation so that more sustainable practices have an advantage. Food and grain prices may be doing this already but it would be nice to have a concerted policy for planning. Lastly, westerners probably eat too much meat but much of the world doesn't get enough meat protein, but that is an economic problem (distribution of resources) that I don't have a solution for no matter how meat protein is created.