In most societies, incest is taboo for ethical, psychological, legal, and biological reasons. There is a power gap between families, which can make it difficult to really agree to, and there is a high chance of problems such as trauma or family collapse.
It is legally prohibited in most countries, and the risk of genetic diseases increases when a child is born. Of course, some argue that adults have the right to freely choose, but there are still many negative views socially and legally. After all, incest is not recommended considering the impact on individuals, families, and society.