https://blogs.timesofisrael.com/the-jerusalem-parade-and-homosexuality/
It is here that a difficulty arises for secular philosophy and ethics. On the basis of which rational principle should a homosexual relationship be permitted but incest forbidden? For the same reason one could ask why a normal hetero-sexual relationship is permitted and even encouraged? What, after all, are the moral grounds to permit such a relationship? Perhaps every kind of sexual activity should be forbidden and considered to be unethical. This is what the famous Danish thinker and father of Religious Existentialism Soren Kierkegaard seems to be claiming. (The Last Years- Journals, 1853-55) when he argues against marriage.