The problem is, if you take that road, there's no way to come to any kind of agreement on a moral issue, whether it's vegetarianism, abortion, murder, etc.
I realize this. That doesn't demonstrate anything, however. It's completely true, but it only highlights an inconvenient truth if morality is relative.
There are far better arguments against moral relativity.
I'm pretty much a utilitarian, but this is a gaping hole in your argument. Until you provide a definition of morality, and give (or cite) an argument for why it's accurate, any argument you base on morality is completely meaningless to anyone thoughtful enough to see that almost all of your argument hinges on how you define morality (which isn't very thoughtful).
Even with utilitarianism, the consideration as to which 'beings' should be included in the set of things we calculate utility on is completely arbitrary.
It is arbitrary indeed, but I think it makes sense to calculate utility, at the very least, for anything that thinks, has emotions, and feels pain. Also, even if you only include them as a minor point, there are FAR more of them than us, and their losses far exceed our gains, so it makes a difference.
If you don't agree, then we've found the core issue where our opinions diverge.. the rest is all just noise.
Also, utilitarianism is fundamentally broken. Any system where you can justify genocide is bad!
