What's the difference. Two consenting adults is the argument that is used to justify homosexuality. Logically speaking, incest should also be legalised everywhere in that case. He has a good point.
Nour
Many countries ban incest sex not out of moral issues, but with reproductive issues involving high birth defect rates. According to Theists and their Adam/Eva narrative, the world was populated via incest. Having said that, there are countries that don't have contrarian laws to incest. For example, Mainland China, Ivory Coast, France, Belgium, Luxembourg, Portugal and Spain have no laws banning incest.