There is no firm scientific evidence that homosexuality is a common condition found in nature, especially not in mammals. Cave drawings in France and Spain included images of sexual acts between men and women. There were, however, no such drawings of homosexual acts. This strongly suggests, if not proves, that the normalization of homosexuality is a modern social invention. Today, in an effort to maintain political correctness in America, biologists and researchers suggest that it is found in nature by stating that a few animals exhibit homosexual "behavior". This is hardly a confirmation of homosexuality in nature!
I think that to say it does not occur in other animals makes it wrong is an ignorant statement. Humans are a species different from all others, exemplified through out intelligence and accomplishments. There are many things that could be referred to as a "modern social invention" including the abolition of slavery in almost every country, equal rights for women, and equal rights for different races and ethnicities. "Modern social inventions" are what pushes us forward as a species and we need to keep progressing and adapting to the world around us and our own people. I think to restrict the actions or rights of others in the case of homosexuality is no different.
To say that it does not occur in other species makes homosexuality an unnatural act. Whether it is "wrong" or not is another issue entirely. Slavery, race and gender issues are not modern issues and have been around forever. Women's rights since the caveman drug his mate by her hair into a cave for sex and slavery since he became dominant in his tribe. Homosexuality is a sexual perversion desired by some small segment of most every society since written history began and most of those societies have shunned the practice. Nature is not subject to change because of popular social acceptance, at least not yet. No matter how intelligent man becomes he can never separate himself from nature, can never make something man-made natural and should never think he is above the natural universe. He is so deeply bonded to the natural world that he even attempts to justify his "invention" by trying to find his sexual perversion in the animal world to which he belongs. The acceptance of homosexuality as natural is a result of man's arrogance of mind. Is it so surprising then that he pollutes his atmosphere, pillages the land and the oceans and yet justifies his violations? I think not.
Homosexuality is a natural human act. Since being homosexual is not a choice, it means it is biological- something you were born with. This makes it 100% natural. If you are born with something, you have no control over it, and therefore it should eliminate all arguments of naturality. You have no argument.
You refer to man's intelligence and accomplishments. Do they really exceed those found in nature? Yesterday I found a birds nest while I was cutting grass. I marveled at how an "inferior" species accomplishes such a feat. Is it really any less impressive than a man building a bridge? While looking at the nest I noticed pieces of fishing line among the outer pieces of sticks and grasses in the nest. It made me feel bad. Then I noticed some duck feathers inside the nest. How does this bird know it is better to line the nest with the down of ducks? There are thousands of examples like this in nature. Is the bird making conscious decisions or choices? Of course not, it is evolutionary in nature. It is behavior designed to insure the species survives. Every act by any species in nature is beneficial in some way to survival. Sex to procreate is a natural part of this survival mechanism. Every homosexual person on this earth has a mother and father. Science can create a person in a test tube but would you call that natural? Would they be able to guarantee the test tube person would be homosexual? My point is simple. No one in biological science has proved that a person is born that way, while there are a few suggestions of genetic causes, most of the scientific community would call that a mutation simply because of the evolutionary implications. As for the animal world you will always find the term "homosexual behavior". No one in science will say that any bonobo ape IS homosexual. It would be like saying, if my male dog humps on my leg he is homosexual if he chooses my wife he is straight. Unless of course my wife happens to be a man, but that's a whole other issue. This is how ridiculous the argument has become. Even if you discount the evolutionary argument there is no way you can consider less than 2% of worlds human population normal or natural. At least not by the current definitions of the words. I don't care what two consenting people do, even if they are not adults. Just don't insult my intelligence or the animals by calling it natural.
You made an interesting point in saying we have sex to have babies. Yes, we do, but the majority of the time is for pleasure. I do disagree with your statement that it is no less impressive for a bird to build a nest than man to build a bridge. A nest is impressive, yes, but after hundreds of years of learning and adapting, we have improved bridges to withstand hurricanes and earthquakes. This shows the ingenuity of man. I hate to use the "m" word in referring to homosexuals, but it is a valid argument. Just like cleft lips, or extra toes, homosexuality is a "mutation" that people are born with. Although it occurs more often than the examples it similar in naturality. Plain and simple. If you're born with it or do not choose it, its natural. Get over it.