Humans are, properly, a different and higher moral class than non-rational animals because we have the kind of consciousness that makes us morally responsible for our actions.
It is impossible for anyone to continuously exhibit the consciousness which makes us at once transcendent and rational and moral, and so we impute rights and responsibilities to others simply because they are like us (others of the same species, others with the same nature.)
If we brought into being quasi-human entitites which might or might not be truly persons, we would have to test each one to see if it (or he or she) were in the human community, or not. And this is impossible, because the required consciousness does not manifest itself unambiguously and continuously. It's not easily or perfectly testable.
The result would be, not to elevate new entitites to the human status, but to destroy the foundation for human identity and dignity, and thus degrade all.
I realize that every sentence I have written could be expanded into a paragraph or a chapter; and that each will be disputed by people who lack either the ability or the inclination to read paragraphs and chapters.
So, here's my shorthand argument: Aldous Huxley ("Brave New World") and C.S. Lewis ("The Abolition of Man") ought to be attended to carefully by anyone who want to think through the new humanism issues (post-, quasi-, and trans-humanism).
It encourages me that Huxley was an agnostic (though "mescaline mystic" or "humanist dystopian" comes closer to his belief); and Lewis a pagan-turned-orthodox-Christian; it makes me think that the pro-human-dignity argument really is universal, ultimately affirmed both by natural law and religious insight.
Excuse my rambling. Nuff said.