I've thought a lot (and I mean, a *lot*) since you posted this.  I didn't want to reply immediately.

If you were an extraterrestrial cataloguing different life on Earth (cue an extraterrestrial discussing humans with David Attenborough's voice, or that's how I like to imagine it), you would conclude we were omnivores by observing our behaviour.  Extraterrestrial David Attenborough would likely also observe that the most plant-based populations are the healthiest, live the longest, the most vibrant in old age, etc.  And I think you and I broadly agree on all of the above.

I think the "correct" answer biologically is that both humans and dogs are omnivores.  Dogs are omnivores, that lean more towards carnivores.  They can live on an entirely plant-based diet, or an entirely meat-based diet.  Unlike humans, dogs lean more towards carnivores though because they have a larger number of physical characteristics in common with carnivores, and like carnivores dogs can create vitamin C in their body (which is why they can exist on a meat-only diet).

Humans are omnivores as well but, unlike dogs, humans lean the other way and lean more towards the herbivorous side of the equation.  The longest-living and most vibrant human populations are the eat the most plant-based diet, and humans can't create vitamin C in our bodies so we need to eat plant-based nutrition or we die of scurvy (among other diseases).  A human dying on a meat-only diet doesn't even take long, there are stories of people sailing from Europe to North America and eating only fish and cured meats, and some of them dying before the crossing was finished!  There's a famous story in Canadian history of a crew of Europeans arriving in what is now Eastern Canada, half dead from eating only meat for the voyage, and it being the middle of a typically bad Eastern Canadian winter.  Nutrition wasn't well understood in those days (they hadn't even discovered food had nutrients yet, or that germs exist, etc.), and even if they had understood nutrition, what plants were they going to find under the Eastern Canadian winter snow?  The local first nations (aboriginals) in Canada showed them that they should eat bark to get vitamin C and other plant-only nutrients to cure the diseases that they were dying of, and that saved the crew.

And while dogs have physical characteristics that lean more towards the carnivorous side of the equation, the majority of human physical characteristics lean towards eating plants.  For sure you can find points that lean one way, or the other, about human physiology, but the *majority* of our biology points towards us being best adapted to eating plants.  And if we don't eat any plant-based nutrients at all, we'll die in a hurry.

So we are omnivores and can include meat in our diet.  We can even live off of a lot of meat, for a while at least, before our odds of heart disease, cancer, diabetes, hypertension, etc., finally catches up with us.  But we're not as omnivorous as dogs, as dogs can seemingly live equally well off of a meat-only or a plant-only diet.  I think dogs are possibly the most omnivorous beings on the planet, actually.  We can live extremely well off a plant-only diet (possibly better than an omnivorous diet), whereas we can't live at all on a meat-only diet without any plant-based nutrition.  So we're omnivores I guess, but I think we undeniably lean more towards being plant-eaters.

As for fish, the biggest problem with it is the ocean pollution.  There is strong evidence that fish-eaters are 2-3 times more likely to get diabetes than vegans, for example, and the problem might not be the fish themselves, it might be the pollution in the lakes, rivers, and oceans.  In fact, some recent research suggests that 95% of the heavy meatls, PCBs, and other environmental toxins that people consume come from animal fat (as it bioaccumulates up the food chain and gets stores by animals in their fat).  In this instance, fish is the worst offender as fish live their entire lives in tremendously polluted waterways, whereas most other meat people eat is factory farmed and not exposed to as many environmental pollutants.  So fish is worst for environmental pollutants, red meat/dairy/eggs is worst for heart disease, poultry worst for communicable diseases (salmonella, bird-flus) and has been recently implicated in a raft of human diseases, there just isn't any meat that seems any safer than any others.  Sure you can eat meat in moderation, but that's like smoking in moderatio...  you can do it, and you might even get away with it, but it's probably not ideal, and it's definitely not an evidence-based decision.  The people behind creating the DASH diet for the American Heart and Stroke foundation travelled the world and studied human populations, and they concluded that eating entirely plant-based is ideal for human health.  They then created the DASH diet that recommends people eat some chicken and fish.  Why?  They're on the record as admitting that they watered down their recommendations from what was ideal for human health to something they thought the American public might accept.  A shame, as I'd have rathered they were honest with me about what was ideal for my health, and let me make my own decisions about whether I followed the ideal recommendations or made some sort of compromise.

There is a distinction to be made here.

Smoke contains substances that are have a slight chance of modifying DNA. So, each cigarette adds to the probabilities of getting the likes of cancer. The substance also depositis on alveols, each time killing off a few cells, making breathing just a bit harder.

Diseases related to the consumption of meat (heart, mostly) appear due to an imbalance. The substances are NOT inherintly harmful to human cells, and are naturally produced by our body. There IS a certain range of concentration in which our systems function perfectly normally; more or less inside this range is not better/worse. Going above (or below) is what creates problems. 

This is the same as any other substances. Example: saturated fats in a cell membrane reduce the cell's exchanges with its environnement, while unsaturated fats increase it. Both too much or too little exchange can create problems. There is a range where these problems do not occure. The body does have a cerain flexibility on the ratios between essential nutriments.

If it's the first piece of meat you are eating in your life, eating a chicken has straight up no consequences on your cardiac health. It's a slight shift in the balance. As long as it doesn't exceed any threshold, there is no reason to believe it to have any negative consequences, at all.

This is a case where poison is in the quantity.

Another thing: I think you might be overstating the dangers of bird flu/salmonella. Both can be detected and treated fairly quickly. In a modern western society, they can cause big dammage, but that's purely monetary. It gets detected (10-100 cases, at worst, and as said, after a week of modern medication, they are all fine) and the producers consequently are forced to get rid of all their livestock. 

Agree on most of this. 

A bit of nitpicking, but I do believe that that has much more to do with the efficiency of a plant-based diet than the overall effect on a person's health. 

Meat just cannot be produced in the same density. If you want alot of people on little land, it needs to be grains/tubers.

It's difficult comparing meat-based and plant-based societies, since their overall way of life is so different that main detterminants are outside factors for anything you want to compare.

If you look at a pure average age, for any given period of time, the former will have the advantage - plant-based societies lived in larger numbers, closer together, which makes the population more prone to diseases and outbreaks (which killed off 90% of the population in th old days.), while hunters and herders lived in smaller groups that travelled around.

Because of the beforementionned reasons, the shift to an agricultural lifestlye (which massively reduced meat intake) actually reduced global life expectancy quite significantly. (I know, wikipedia. But he article is well-sourced.) https://en.wikipedia.org/wiki/Life_expectancy

I am part of the hardline pro-meat camp.
I wouldn't be able to give up meat for a
2 weeks, even less a lifetime.

worth it? No because you would need a lot of supplements and also vegetable protein is really crappy in comparison to MEAT protein.

I've always followed what my body craves, which has served me well. Except that fish has become so expensive we hardly ever buy it anymore. My father in law sometimes goes fishing in summer which he shares. Yet who knows what those fish eat. Meat is easy and quick to prepare, tastes great and provides most of what I need.

As for morality, sure it's bad to kill another (maybe) sentient being for food. Yet compared to nature cows live a relatively relaxed safe life. Seeing them lounge in a meadow compared to the horrible squealing I sometimes hear at night along the river, ending up as roadkill, freezing or starving to death. Being spared old age or getting eaten alive doesn't seem so bad to me :/

Economically, well as long as I can afford it I'll keep eating meat. It's not that vegetables, nuts and what not are any more affordable. If meat is so much more expensive to 'make' how come a vegan lifestyle is more expensive...

Anyway, I'm not much of an adventurous eater. I always have bread with peanut butter for breakfast, and just made a big pot of macaroni yesterday I'll be eating from for a couple of days. Easy, quick, more time to play games :)

As recent research more and more suggests that plants may have vision and consciousness, there are no arguments for being vegan. If you do not want to harm another form of life you are allowed to eat artificial food only.

I'll probably never become a vegan, but I get worried when I see how many resources are used to farm meat, that it makes me think just how much land and produce we could save by becoming vegetarian. Of course, I guess I don't have much of a say, because I still eat meat.

I'm not a vegan/vegetarian, but respect the habit of some of my friends who are vegetarian. Vegan is not really healthy as you will even miss more vitamins and minerals then when you would only be vegetarian. I would advise to just eat a little less meat and/or replace most meat with fish, when you eat around 2x a week meat and the rest with fish (or  vegetarian meal) I think this would help and makes sure you don't eat too much meat (because that's also not healthy) but still don't really need to take additional vitamins and minerals.  As you might know in nature humans need meat (not much) but otherwise we wouldn't not have evolved as we did now. I know quite some vegetarians and they all need to take extra vitamines (mostly B12) but also others and minerals for iron and calcium.