starboard5 saidI get mighty weary of hearing the word "natural" thrown around by people with the intelligence of a clever dachshund (and I'm doing a disservice to dachshunds everywhere, in this case.) It's "natural" to develop tooth decay and unnatural to drill holes in your teeth and pack them with amalgam mercury fillings. Examples are endless; the point is, there's little that's "natural" in the way we live. If we lived a completely "natural" life, we'd still be hunters and gatherers with a life expectancy of 25.
And if homosexuality is unnatural, why has it survived natural selection? But I might be weakening my argument here; clearly, simple-mindedness survives (and thrives) as well.
Easy answers - first, to Evangelicals there is no natural selection, it's all Creationism, so your whole second line of reasoning goes out the window. Nor are homosexuals part of God's design, it says so in the Bible (if you know how to cherry pick and avoid reading the other forbidden stuff the majority of people like to do in the 21st Century). We choose to be homos, and thus are unnaturally defying God and committing a sin.
See? To quote King George V in the movie The King's Speech
: "It's easy when you know how."
Oh, and BTW, the correct answer about why gays exist is indeed natural selection. But first you must understand that humans have evolved culture, as important to our existence as our teeth you mention, and that we are social creatures, not the rugged individualists some heroically pretend are not dependent upon other humans in any way.
We are gay not for ourselves, but for the community. Except the community got fed some lies by ignorant, subsistence-level sheep herders over 3 millennia ago, and by quirks of history, their conflicting & confusing version got written down to survive to modern times. (And no, it wasn't God's will, or else the Hebrew God is as inconsistent and contradictory as his Greek counterparts who lived on Mount Olympus)