But where do you draw the line? How do you determine if the public consents?

Here we get into the specifics of the social contract. By partaking in society, we are bound to the contract, but we also have a hand in writing this contract. Naturally, the world's largest ongoing set of negotiations has millions of iterations, has started thousands of wars, and will never end. Yet, by choosing more reasonable terms, more people will be content with the contract, leading to larger more stable societies. How we choose where to draw the lines is of course central to this issue. This is one of the social technologies I mentioned; as we gain a better understanding of humans, systems, societies, and legislation, we can negotiate a contract based on more universally applicable facts, stronger predictions, and more objective reasoning. There will always be grey areas though, and that's why it's a negotiation.

If you sup with someone negotiating a change, you admit that the change means less than the meal to you. If that change is to accept pineapple pizza as undeserving of ridicule, then meh, ridiculing a pizza topping isn't as important as sharing that meal. But if that change is to eat poor children, then you have valued the lives of children, classism, and unrestricted cannibalism at less than a meal. You could contest this change to make up the difference, but that is a mountain and a half to talk your way out of, depending on how fervently they support it. Anything short ends up being approval, which is why you don't sup with people who want to take away people's rights.

Consumption of human flesh is theoretically A-OK,

According to whom?

According to me, but I invite you to find anything intrisically wrong about the consumption of human flesh by humans. Not closely related problems like encouraging killing or an instinctual revulsion, purely the consumption and it's intrisic completely unavoidable consequences. Genuinely curious.