Certain actions such as incest and rape may not be biologically and socially advantageous and so in the course of human development have become taboo. But that does absolutely nothing to show that rape or incest is really wrong. Such behavior goes on all the time in the animal kingdom. The rapist who goes against the herd morality is doing nothing more serious than acting unfashionably, like the man who belches loudly at the dinner table. If there is no moral lawgiver, then there is no objective moral law that we must obey.
