I can help draw the line as a little thought experiment, but naturally, it's not going to be the clear cut line. I'm just one guy using Google and what little I know.
Me vs the norm:
The line is usually given on animals that seem normal to consume , and animals that are typically viewed as close companions. Animals that are lucky to bond with a human are moreso the "pets".
The line though is still hazy. I have a friend that loves horses. When I heard Ikea had horse meat in their meatballs , I was curious if I ate horse and was tempted to try horse meat if legal . My friend however, was appalled and did NOT share the same view as me.
I'm also a little against the norm, as since many people think dolphins as friendly and intelligent creatures. I don't think of intelligence as a big deal for the most part , as there's "food" animals that can actually be pretty smart too for what they are. I don't actively look for it, and this is likely an unpopular opinion, but if I went to Costco and was offered a free sample on legitimate dolphin meat , I might actually try it. I know alot of people would be disgusted and reasonably so.
When it comes to some really obvious animals like dogs, cats , hamsters , whatever , Im also disgusted at the thought of eating them. They're friends. However , even culturally , there's can be a difference where they're viewed as edible.
Norm vs culture and religion:
Continuing off the dogs and cat thing, most of us would seem appalled to eat an animal that's meant for companionship. However , look at a flipped example. Cows are the "norm" for food, but in India , Cows are sacred. It isn't like some other animals that we would be grossed out to try for example, them being dirty or weird looking , it's because cows in India have a religious presence that I believe was based on them being gentle in nature? Someone Hindu would be able to explain this better , I don't want to offend anybody by mixing up the reason or being wrong on why.
Norm vs veganism (based on what I looked up. Any vegans here , please add your input)
Veganism appears to be based on sentience mainly. By definition , things that can think, display emotion, have desires , and the capacity to experience and perceive.
When it comes to humans , dogs , cats , etc , pretty clear that we all are sentient , hence it's not vegan to eat said part of us. Plants , given things like a carrot can't be sentient , are perfectly fine.
One thing that puzzles me is veganism seems to be against animal products as well. I'm not talking about "beef jerky is an animal product" thing. Say a chicken lays an egg that wasn't fertilized. It's not sentient , nor would it even be capable of sentience ever , but it's not vegan , but I can see the correlation that if it WAS fertilized, then it would be.
Why not milk though? Killing and eating a cow is obviously not vegan. However , milk is produced by a cow in the udders. Milk is not sentient, has no sentient equivalent, and could never achieve sentience. If we also follow the "central nervous system" thing, it's obvious milk has none. If we even add that animals shouldn't be harmed for it , milk actually seems pretty vegan. Say you're not wanting to cause animals harm , so you won't milk a cow because you won't drink milk. Milk producing cows actually need to be milked, so much so actually, that they experience pain FROM lack of being milked. I think it's ok to have a vegan milk a cow that's in pain from not being milked , because they aren't consuming it. However, why can the vegan not consume milk when doing so would cause actions that actually benefit the animal ?