1. As I'm researching the question about whether or not animal protein rots in the gut, I came across this really interesting (albeit somewhat gross) article about digestion: http://www.gnolls.org/1444/does-meat-rot-in-your-colon-no-what-does-beans-grains-and-vegetables/
2. Is white fish easier to digest than red meat? How about beef vs. salmon (it contains about the same qty of protein and fat per serving)
3. If by chance fish is easier to digest than beef, is it because of the fat content or something else?
Nothing "rots" in the digestive acids of a healthy gut. Food - red meat included, vegetables and legumes and starches included - is broken down and metabolized. If there is a problem with stomach acid secretions, you will have issues, but a healthy intestinal tract can handle our omnivorous diet - we are built to digest meats and veg. What isn't digested is sent out the rectum in a bowel movement.
Fish is more tender than grassfed beef, sure, that doesn't mean you can substitute one for the other - and don't forget the required organ meats as well. It's not just about muscle meats.
I'd rather eat more beef than fish, though I do eat wild caught fish. I certainly would avoid farmed fish as much as possible, and I'd avoid CAFO meats as much as possible too.
Mr. Stanton does know his shit. There's nothing gross about it, it's science. If you're squeamish about digestion, how are you going to research it? Every creature eats and digests things in its own ways, even plants and bacteria have "waste" products. It's better to know and understand how things work than not to know.
There are loads of factors that go into digestibility. Heck, the most digestible diet is the SAD, where there's so much processing done that digestion is quite efficient (too efficient perhaps!) Raw foods are less digestible than cooked. Whole foods are less digestible than phsyically processed (i.e. grinding) foods. Meat with lots of connective tissue is less digestible than meat without connective tissue.
Why do raw vegans tend to look skeletal? Because they cannot digest their diet, much of their food being lost as waste, because humans did not evolve eating a raw unprocessed diet.