'Sexual orientation' is an absurd term. There is normal sexuality, and abnormal sexuality. Normal "Heterosexual" sexuality is of course the only way to pro-create, virtually all science shows that pro-creation is the only natural point of sexual attraction. This is true whether you choose to pro-create or not. The want and desire to have sex does have a natural purpose. It is not a racial, white and black thing.
The thing you're dismissing is
human nature. Humans generally have an instinct to do whatever gives them pleasure, regardless of whether it's "normal" or not.