Edit: Surprised at all the vegans in this thread. I didn't think there were so many of you. I'm glad you care so much about animal rights, that you're willing to forego eating them and using products made from them.
If you're not vegan and have moral objections for this, maybe you should look at yourself first and all the animal abuse you sanction by eating animals and using animal products.
Did you know dairy cows have to be pregnant to produce milk? They're artificially inseminated throughout most of their lives. I hope everyone complaining about this also complains about ice cream and cheese. Or else they would be hypocrites who just want to blame others but never look at themselves.
It's not arbitrary, though. It is just hard to define. Ethic theory uses certain axioms that aren't subjective. I am not talking about your moral values, but whether or not certain behaviour is ethical or not.
As a drastic example, driving over a person because it is faster than driving around them. We can certainly decide for some cases whether that is ethically good or not. For the harder to decide cases it's again just a matter of not knowing all variables. If we would know all variables, we could put each reason for driving over a person on a scale of "ethical goodness". Since we have certain axioms in ethics you can logically conclude a result for all ethical questions (in theory).
This is not more made up than mathematics are made up. The quantity (not the mass!) of objects, for example, is also just a thought we put onto objects. It's not in the nature of objects to have a quantity. And if we didn't had an inherent concept of mathematics, quantity would not exist for us. In that way, it's different from gravity and other such physical realities. It is the same with ethics.