Of course, the most fitting ideas always arrive AFTER the conversation's moved on to gay rights, but you know something? Survival of the fittest, this often misinterpreted and often misrepresented concept from Darwin's theory of natural selection, is a cliche at this point, and bringing it up misses the larger point.
Survival of the fittest in nature was what enabled humankind to evolve from "lower" animals (lower in the sense of brain development; no species has a prefrontal cortex quite as well-developed as the human sort). We're already the fittest species on this planet, urban legends about cockroaches nonwithstanding. After all it took for us to become what we are, I think we owe it to ourselves and to the world that bore us to be as human as we can be.