It seems generally agreed that (even fresh, unprocessed) red meat is at least somewhat bad for human health. But why exactly is this the case? It's difficult to find any attempt at an explanation, especially for anyone not too familiar with diving deep into the medical literature.
Is it because the myoglobin in red meat is carcinogenic or otherwise harmful? That would seem to be the most direct explanation, and there are a few studies on this, but they don't seem to be widely cited in discussions on health effects of red meat. If that is true, would it mean that one should avoid dark, myoglobin-rich fish and poultry such as tuna and ostrich just as much as mammal meat?
Is it only because red meat tends to be more fatty? In that case, would leaner cuts of red meat have the same health effects as white meat?
Or is it possible that there is some deep genetic reason why the flesh of mammals is more harmful than the flesh of birds and fish for human consumption? (I had wrongly assumed this was a strange idea, but acvill's answer gives a plausible explanation.)
