Saying that homosexuality is wrong because it is unnatural is ridiculous.
1. The descriptive laws of nature. The laws of nature do not support homosexuality because it is not believed to be a act that takes place in nature.
Homosexuality does in fact occur in nature. There is a such thing as a homosexual animal. You are not obligated to follow the laws of nature and there is no real consequence for doing so. It is impossible for a homosexual to be violating the laws of nature.
2. The artificial as a form of the unnatural.
Saying that homosexuality is artificial and not essential to human life.
That which is artificial is not always bad. Without artificial devices many lives would not exist because of diseases that these artificial devices cure.
3. Anything uncommon or abnormal is unnatural. Homosexuality is not the "normal" thing that people choose, which makes it uncommon and therefore unnatural.
Just because a behavior is uncommon does not mean that it is condemnable. There is no reason to suppose that uncommon behavior is morally wrong. There are many things that are abnormal but that does not make them ethically wrong.
4. Any use of an organ or an instrument that is contrary to its principal purpose or function in unnatural. Homosexuality is promoting the misuse of ones sexual organs. In this misuse the organ is no longer doing the function for which it was made.
You cannot condemn homosexuality on the basis that it is contrary to its purpose. Many body parts are used for something aside from their primary function. Such as teeth that are used for tearing a package open. We would say that teeth are to be used for chewing and nothing more. But using teeth to open something is not morally...