I wouldn’t call the argument a bad faith argument. Perhaps there is a line somewhere, but ultimately, his argument is that “two people who have an increased chance of passing on genetic disorders can’t have children/have a relationship”.

For most people, when asked this question outside of the incest framing, would argue that the state has no role to dictate that line. The slope is too slippery and screams too much like eugenics. It’s only within the social taboo of incest do people think that argument is acceptable.