That's what the difference is, in a nutshell (it's what I posted originally).In essence, you don't NEED to eat animals to live, this is true. Based on pure necessity, neither are up there. But it's natural for us to eat other animals to survive. It's not as natural for us to have sex with other species.. so our brains are engineered to think this way for the most part.
However, evolutionary efficiency is blind. As a society, we have emergent attitudes to things like this, but it is not based on principles. It is, in fact, the other way around. We justify wholesale slaughter of living beings because of this bias, while condemning others. It is the foundation of our principles, these biases.
It is a logical double standard, if we wish to believe we are more than mere animals. Otherwise a naturally formed society would be... very animalistic.