This is a question that logically doesn't work. How would one prove that any sexual orientation is natural? It is also not necessary to prove anything. Those who wish to say that being gay is unnatural are the one who have the burden to prove their point, not us.
As for me personally, the desire to have an intimate relationship with another guy is strong. The desire to have an intimate relationship with a woman is zero. It comes naturally. I didn't choose it. It is just there. That is the proof that I need.
If you are speaking from a Biblical perspective, the same also applies. You can't prove that homosexuality is unnatural or sinful. The Bible doesn't say that it is. Also the old argument, "It's sinful because the Bible doesn't say it is OK." doesn't logically work either. Then everything in the universe would be sinful if the Bible didn't mention it.
For I am convinced that neither life nor death...neither the present nor the future nor anything in all creation will be able to separate us from the love of God that is in Christ Jesus our Lord. Romans 8:38-39