The argument runs like this:
- Homosexual behavior is not procreative.
- Non-procreative sexual behavior is not natural.
- Unnatural sexual behavior is immoral.
- Homosexual behavior is immoral.
1. Homosexual behavior is not procreative. This premise is true. I am unaware of any anatomist, psychiatrist, biologist, anthropologist, sociologist, parent, gay activist or plumber who would argue against this premise.
2. Non-procreative sexual behavior is not natural. This premise is false. Psychiatry, anthropology and sociology all show that sexual behavior is far more than for procreation. Sexual behavior has social and psychiatrical functions in human society. This can also be shown in other primates. In nature, primates exhibit sexual behavior, even homosexual behavior, which is not procreative. Therefore, is is very clear to me that non-procreative sexual behavior is not necessarily unnatural.
3. Unnatural sexual behavior is immoral. I have not seen any support for this premise. Some natural sexual behavior can be shown to be immoral in human society. Forced sexual submission, rape, does occur in a number of mammalian species in nature including humans. It is immoral since it violates the will of one human by another. But, I have not seen any reasoning or logic which shows that unnatural sexual behavior is immoral. If someone were to show that unnatural sexual behavior is, in fact, immoral, then there might possibly be many common sexual activities which might be thus condemned. Could one say, "Lips are made for keeping food in your mouth when you chew, closing off the airway when you breath and forspeechh. Kissing is not natural. It is wrong to teach my children that kissing is a valid life choice. I don't want my children to learn to tolerate those left-leaning-bubble-headed people who have a kissing agenda."
Conclusion It cannot be shown that science proves that homosexual behavior is immoral.