Note: I don’t necessarily condone any view here. Like many of my previous posts, I’m literally just reporting some useful passages from published academic texts on a particular ethical and legal question.

Content warning:

Incest, obviously
Legal and ethical perspectives on sexual abuse and rape

Maryanski and Turner 2018, Incest, theoretical perspectives on

The spread of Catholicism in the West dramatically increased the day-to-day salience of concerns over incest. […] a sin that, not surprisingly, only the Catholic Church could absolve. The result was an increasing awareness and anxiety over incest—a concern that has never disappeared and, indeed, has been reinvigorated in the present.
Confusion over how to conceptualize kinship in early anthropology and sociology led to seemingly bizarre speculations by early scholars; yet, much of their thinking still provided important theoretical leads for more recent theories.
if anything, that the rule against incest in the nuclear family is so widespread as to be almost universal. The goal, then, is to explain why such is the case.
the one well-documented exception to the universality of the incest taboo is evidence of brother–sister consanguineous marriages among royals and among commoners in ancient Egypt during the period of Roman rule
more recent data on brother–sister incest in the literature on contemporary child abuse have documented both voluntary and forced incestuous relations among siblings, but these data are still ambiguous about how “voluntary” such relations have been because they have generally been found in highly pathological families where drugs and sexual abuse are evident
Explanations from sociobiology: Biological theorizing has always assumed the existence of mechanisms installed in mammals by natural selection to avoid the effects of inbreeding […] Biological theorizing is not, however, well suited to explaining more purely cultural processes
Explanations from evolutionary psychology
Explanations from sociology
Explanations from evolutionary sociology
Anthropological explanations

Green 2019, Incest, The Palgrave Handbook of Applied Ethics and the Criminal Law

at least among liberal-minded scholars there is a growing consensus that current prohibitions on consensual adult incest are, if nothing else, ripe for reconsideration
if the Constitution protects the right to same-sex marriage, should it not also protect the right to incestuous marriage?
Juvenile incest
[Analogy with statutory rape, which is usually criminalized] Virtually all legal systems agree that adults who have sex with juveniles should be subject to criminal sanctions, but explaining exactly why this is so is more difficult than it might at first appear. [… a more plausible justification would rest on the notion that when adults have sex with juveniles, there is, in virtue of their age difference, such a significant potential for exploitation that such conduct should be categorically banned.]
we can still ask if there is any significant moral difference between cases in which an adult and child are related and those in which they are not […] A reasonable argument could be made that, other things being equal, it is typically even worse for an adult to have sex with a related juvenile than with one who is unrelated. Often, the adult will have a special duty of care to the related juvenile. […] Moreover, the harms done to related juveniles are even worse than those done to unrelated ones.
Adult incest
we consider three possible rationales for criminalizing adult incest
(1) it creates a significant risk of birth defects […] One obvious problem is that only a particular subset of all cases of incest poses any such risk—namely, those involving consanguineous fertile partners of the opposite sex having vaginal intercourse without adequate contraception. Under this rationale, incest laws that apply to cases involving people who are not having sex of this sort are clearly overbroad. […] (and the non-identity problem!!!! wow that’s a thorny ethical dilemma) […] non-incestuous couples are completely free to conceive children even when they are aware of significant risks of birth defects […] yet any attempt to impose criminal sanctions on parents who failed to prevent or terminate such pregnancies, or even obtain genetic testing, would surely be regarded as a gross invasion of privacy and an unacceptable limit on sexual autonomy and reproductive freedom.
(2) it confounds well-established family roles and contributes to family conflict […] The problem is, we really do not know the extent to which consensual incestuous relationships between adults affect family stability. As we saw above, there are virtually no data on the incidence of such relationships, their dynamics, or their psychological impact on participants and their families. […] In a liberal society such as ours, people are free to engage in all kinds of intimate behaviors. […] They can be rigid disciplinarians or overly permissive. They can be cossetting “helicopter” parents or parents who leave the job to paid nannies. Any of this conduct may cause great dismay, and even long-term estrangements, within their families. Yet there is little the criminal law can, or presumably should, do about any of it
(3) the putatively consensual nature of such conduct is often illusory and tends to mask deep structural inequalities in family life […] though it might be reasonable to speculate that sexual relationships between parents and their children are likely to be coercive, it seems dangerous to base such significant liberty-limiting legislation on non-empirically verified speculations
An even more serious problem concerns the lack of empirical data we have concerning incestuous relationships involving adults. If one reads the literature on adult incest, one is struck by the extensive reliance on a small body of anecdotal evidence that may or may not be representative of such cases more generally

Subger 2014, https://www.project-syndicate.org/commentary/should-adult-sibling-incest-be-a-crime-by-peter-singer-2014-10

link
should we allow our judgment of what is a crime to be determined by feelings of repugnance that may have strengthened the evolutionary fitness of ancestors who lacked effective contraception?
https://www.project-syndicate.org/commentary/should-adult-sibling-incest-be-a-crime-by-peter-singer-2014-10#:~:text=Last%20month%2C%20the,with%20his%20sister.
not a crime in France, Belgium, the Netherlands, Portugal, Spain, Russia, China, Japan, South Korea, Turkey, Côte d’Ivoire, Brazil, Argentina, and several other Latin American countries.
couples describe the difficulties created by the criminalization of their relationship, including extortion demands and the threat of loss of custody of a child from a previous relationship.
in no other situation are voluntary sexual relationships between people capable of self-determination prohibited
The report examines the grounds on which it might be claimed that this burden of justification has been met. The risk of genetically abnormal children is one such reason; but, even if it were sufficient, it would justify only a prohibition that was both narrower and wider than the current prohibition on incest.
they [research participants asked about a hypothetical incest situation] often responded: “I can’t explain it, I just know it’s wrong.” Haidt refers to this as “moral dumbfounding.”