Could the bold passage be used to protect incest laws? By the very nature of family relationships consent might be hard to refuse. Family members are in position to put undue influence on other members and basically destroy a person ability to legally consent.
I don’t see how it could be argued otherwise, assuming we’re talking about all participants being adults. But someone would have to challenge the laws as was done in the Texas case.
Coercion to gain consent would still be illegal. Manipulating the “family relationship” to make it hard for someone to refuse would weigh against whoever was charged. This decision wouldn’t protect them from the charges of manipulating to gain consent. In fact, manipulation to gain consent would run them afoul of rape laws.
Still, I think you’re misunderstanding the bolded passage. This wasn’t a statement of what the court was talking about, it was a specific disclaimer saying that they WEREN’T talking about those types of relationships. That this ruling would NOT apply to those situations. As with most legaleze there are weasely qualifiers all over the place. This seems to be one of them.