There's a concept of saying you prefer or don't prefer something and a concept of saying people ought to prefer or not to prefer something, and morality is in regard to the latter. The problem is that without invoking religion, there is no way to go from the former to the latter. How do you go about saying that someone else ought to share your preferences and if they don't, then they are wrong, or how do you know that you ought to have the preferences that you do? If you say that everyone ought to not eat meat and those that do are wrong regardless of their opinion, then how do you show that to be the case? You could appeal to other preferences, such as health, or animal cruelty, or the environment, but what if they don't share those preferences? How do you make the case that they also ought to share those preferences and that they are wrong for not sharing them? The only way to get out of this to invoke religion and say that God has given a standard of right and wrong preferences that we ought to share. Without invoking religion saying that someone should share a certain preference is like trying to say that they are wrong for not liking chocolate ice cream better than vanilla. So if you're a vegan because you prefer that philosophy, then that is one thing, but if you want to say that people are wrong for not sharing that philosophy, then you need to invoke religion.