Heterosexuality is not natural, just popular, because some of it's expressions tend to lead into conception and thus passing on of those tendencies to offspring.
Erm, that is usually what evolutionary biologists mean
by natural. Have you read The Selfish Gene? Of course, natural doesn't mean right
There must be some
evolutionary influence on behaviour, otherwise different species wouldn't have such different (usual) sexual patterns. For example, the other two chimpanzee species tend to use sex the way we'd use a polite handshake. Why don't humans?
My favourite book on this stuff is The Red Queen
, by Matt Ridley. The later chapters are basically an exercise in comparative anthropology, except that instead of looking at different tribes he looks at different species. Birds in particular have a lot of different mating patterns, depending on the species' environment.