This is such a silly line of thinking. To call being gay unnatural is clearly a misuse of the word, but the idea that everything that exists at all is natural is nonsense. For example, hot pink is an unnatural human hair color. Even if you color your hair hot pink, and therefore it exists; that doesn't make it natural. So in the absence of another word used to describe something that wouldn't otherwise exist in nature without human intervention, ... "unnatural" is perfectly suited to this task.