For example, you believe anal sex is "unnatural" because it does not lead to procreation. By extension, you believe masturbation and oral sex are equally as "unnatural" since they don't lead to procreation. The hand and the mouth are not reproductive organs. Even In vitro fertilisation, which is the process of fertilising an egg with a sperm outside the body, would be considered a highly "unnatural" act by you.
Most people today use an empirical definition of "natural". Empiricism is the basis of science, and it holds that observation and measurement are the key to understanding the world. As such, the modern definition of "natural" is that which is observable and measurable. Homosexuality is "natural" by this definition because it can be observed occurring prevalently in the higher animal kingdom.
You definition is incredibly outdated and not used by most of the modern world.