Same-sex sexual behaviour is often condemned on the grounds that it is "against nature". Indeed, biology tells us that selection favours those who leave more offspring. But then, homosexual behaviour is widespread - not only amongst humans, but other animals alike, be they flamingos, gorillas, dolphins or bisons. Doesn't this constitute a paradox for Darwinian theory? And is there a connection between what goes on in nature and what is morally desirable? This talk will address these controversial topics.