In your example of people eating crap, this is behavior that only affects the individual. Regardless of how gross it is, when we have existing movements that allow people to do drugs or smoke cigarettes, then clearly there isn't much pressure or a general feeling that crap eating represents some kind of threat or moral panic.

A individual's actions affects the community. Eating poop disgusts most people, if someone saw a person eating poop it would viscerally disgust them and reduce social trust in that community. Also if the person ingesting poop catches e. coli, salmonella or hepatitis A that may warrant a clinic or a hospital visit. People will be waiting in line longer for treatment for ailments that are not caused by poor life decisions. If you have universal health care that is a wasteful tax burden that puts a cost on the entire tax paying community.

But if your society does allow even one individual to mix with another race, then you are giving the invitation to let half-breeds participate in society, since they're not legally barred from voting, or owning property, or running a business etc. You say huge genetic differences are the reason for Whites to ban miscegenation first, but why not Native Indians?

There is already big social costs that deter miscegenation in most cultures. It is not uncommon to hear stories of interracial couples talking about being disowned from one or both sides of the family or feel their relationship has been estranged after announcing to their family their foreign spouse. Since there is big social costs for proclaiming opposition to miscegenation in Western countries, people will act passive aggressively to sabotage relationships whether they are aware of it or not. This might be hyper criticism of the foreign spouse, write them out of the family will or stop inviting them to social gatherings. Women also defer opinions from their group of friends, if all her friends disapprove of the relationship it is less likely that relationship will succeed in the long term. There probably are more variables that I have missed but they all deter miscegenation to the point of making a law is often seen as redundant. Also as some others have stated, to participate in the world economy, countries have to put on a face of political correctness.