We do have agency involved in whether or not to eat animals. Eating animals for humans who are not severely mentally or physically disabled is a choice, as we can follow a plant-based diet as well.

And moral relativism isn't the basis of buddhism. The basis of Buddhism is extending compassion to others. If we extend compassion to animals who have lungs like us, breathe like us, defecate like us, experience fear, joy, sadness, pain, and pleasure much like we do, we would be extending compassion for ourselves and respecting the parts we have in similar with each other. If we willingly take an animals life, pay someone else to abuse them, for our own pleasure, we are also harming ourselves by separating ourselves from our compassion.