EYT:
For one thing, you haven't demonstrated why something being unnatural is the same as it being immoral. Medicine isn't 'natural' - neither's education, or living in a house. Or, on a more related note, neither is contraception. Our civilization isn't natural by any stretch of the imagination, and is all the better for it.
For another, there's numerous scientific theories which between have them indicate a possible natural origin (though admittedly the field is a bit inconclusive). For instance, a tendency has been identified in twin studies between the likelihood of homosexuality in proportion to the number of pregnancies the mother has (the more sons, the more likely the next one is to be gay). (http://en.wikipedia.org/wiki/Fraternal_birth
_order_and_sexual_orientation)
In the 90s, a brain scientist called Simon LeVay also identified a recurring difference in a part of the brain in gay men that wasn't in straight men (though he emphasises this does not prove conclusively, it just indicates the possiblity).
An overview of different theories can be found in the following links:
http://en.wikipedia.o...nd_sexual_orientation
http://www.theatlanti...ity-and-biology/4683/
http://www.webmd.com/...128/is-there-gay-gene
http://psych