Perhaps you still think the comparison is silly. But it’s not. Here are some relevant questions for liberals. I think you’ll be surprised where the answers lead you.
Do you support the right of consenting adult brothers and sisters to marry? If not, why not? What legal or moral principle justifies granting marriage rights to unrelated same-sex couples while denying such rights to brothers and sisters?
Note that the German Ethics Council also held that the prospect of a brother and sister producing children with genetic defects cannot be used as a reason to deny them a right to marry. After all, disabled couples are not prohibited from procreating under German law, even though they have a greater than average chance of producing disabled kids. The same is true, incidentally, under the Americans with Disabilities Act of 1990.
If you do support the right of brothers and sisters to marry, is it because you think there’s nothing wrong with (or even something potentially good about) incestuous relationships? Or do you support the right to incestuous marriage despite being disgusted by the practice?
This question is significant because in recent years some liberals have begun to argue that it is not enough for traditionalist religious believers merely to tolerate same-sex marriage. Instead, these people must positively affirm the legitimacy and goodness of gay marriage as an institution. Otherwise, they run the risk of perpetuating the evil of homophobia.