With the recent Oshi No Ko debacle on incest, now I'm trying to rationalize society's unfavorable opinion on incest. What element in incest makes it bad? If both people love each other romantically, is of the right age and neither was coerced into doing so, then what right does society have to keep them apart?

Okay, someone may argue that the same bloodline can cause genetic deformities and disorders in their offspring. I get that (Hapsburg jaw is a thing after all). But if that's the case, then shouldn't we also frown upon getting together with people with inheritable diseases (i.e., heart disease, cancers, diabetes), genetic disorders, or harmful lifestyles (i.e., cigarette smoking) that increase the risk of offspring suffering due to those inherited ailments/disorders? AAAAAAHHHH

Now I can't seem to find how incest is objectively and morally harmful to society. Does anyone know a strong argument against it?