Nature = noun
Natural = adjective referring to the noun, Nature
Acting in accordance with your nature is by definition natural.
If your nature is to be attracted to the same sex, then making love to the same sex is going with your nature and therefore natural behavior for you.
If your nature is not to be attracted to the opposite sex, then making love to the opposite sex is going against your nature and therefore unnatural behavior for you.
Shouldn't this be an obvious given?