Another argument is that homosexuality "isn't found in animals". Apparenly people need to wake up. Scientists have found it in nearly every species. Even the kid down the street can tell you what the two male dogs do to each other in the neighbor's backyard.
This doesn't mean that just because animals engage in homosexual acts that human homosexual acts are moral because we can compare humans to animals. Animals often eat and kill their own siblings, mates, offspring. It would be a rather foolish argument. However, what this does say is that it's just as foolish to use the reverse argument that "since animals don't do it" (and they do!) "it's wrong". Animals don't buy car insurance and write poetry, but none of those things are on our "abomination" list. Let's not argue whether there are gay giraffes, alligators, lion, etc., or not. It's a waste of time.
Another argument is that if something isn't used with it's principle intent, it is immoral. I use my mouth to speak, eat and drink. That is it's priciple intent that is demonstrated clearly in the Bible's pages by example. However, where I run into trouble is when I brush my teeth and chew gum. Will God condemn me for doing these things? I hope not! :) Nowhere does it say that men like Abraham had Big Red and Aquafresh. Am I sinning everytime I chew on the back of a Bic pen top?
Penis in vagina might seem logical. I agree it is. But, this one use doesn't necessarily designate definitive purpose. The chief purpose of an organ doesn't formulate morality. It only forumalates data frequency and statistics. Breasts are chiefly mammry glands for giving infants milk. If God only wanted them to be used for this, then why aren't women flat-chested until they are pregnant?
Finally, we cannot assume what is "unnatural" (if it even is considered that to begin) to be immoral and imply that natural is somehow "good". One part of the dark side of human nature (sadly) is murder. Some people are born with a genetic predisposition to become an alcoholic, if given the opportunity. Others may become psychopathic serial killers on the converge of antisocial personality disorder.
Storms and earthquakes kill thounsands of people at a time. Cancer takes the innocent lives of hundreds of children every year. This is all natural.
So, after clarifiying this bit, we can see that there is no basis to say that homosexuality is "unnatural".