Can veganism be defended against moral relativist beliefs, assuming moral relativism is correct? I might be missing something here but it seems like veganism almost always requires a non-relativist framework.
Can a case for veganism be made from a moral relativist perspective?
Absolutely. A vegan can simply recognize that their arguments for veganism apply to some people and not others. This is a vegan who embraces philosophical pluralism. Many fine, kind vegans say, it's right for me, and if bacon is right for you, then so be it.
Now, there are MILITANT vegans who think in absolutist terms who believe there is one correct way to think and one correct conclusion, so on. In such case, they may brandish and wag a finger. "Eating animals is killing the planet". "To kill conscious animals is murder". "If you are not part of the solution regarding animal rights, you are part of the problem." Such a vegan does NOT embrace moral relativism. If you recognize veganism as being consistent with militant veganism, it may be the availability bias at work since militant folk tend to create a stronger impression.
So, the lesson to be learned is that one's beliefs about veganism and one's beliefs about morality are two different matters, and that in some sense, whether or not one is a vegan may be independent of whether or not one is a moral absolutist or a moral relativist. The difference in the latter positions obtains not just to the morality of killing and eating animals, but to morality more generally. Hence, religious folk can be morally absolutist (My God is the one, true God!) or moral relativist (Let's be ecumenical. Surely God or gods or whatever superpower would want us to get along and build bridges rather than wage crusades).
Many vegans argue from a relativist perspective, relying upon a person's attitudes towards animal cruelty to reach an absolute claim about how a person opposed to animal cruelty should behave.
If you believe that we shouldn’t be cruel to animals, i.e. we shouldn’t harm animals unnecessarily, and
If you believe the consumption of animal products harms animals, then
Animal products should not be consumed when doing so is unnecessary.