Everything humans do is unnatural, and the meaning of unnatural cannot reasonably be decided by any religious faith or texts

Unnatural does not equal unethical. Moreover, reproduction is not a moral decision - there is no need to have children, or to not have children, we are not in short supply of them and so sexual actions not resulting in child birth cannot be considered moral or immoral, they simply are as moral or immoral as any other form of sex, planting them firmly in the category of "morally acceptable behavior".

Something cannot be judged to be wrong if there is no harm caused by it - homosexuality, therefor, is a morally neutral activity with no potential for abuse that stems uniquely from homosexuality.