freda -

A minor quibble:

incest by its nature is about an adult coercing a child

In most US jusrisdictions, and I would assume the same in Canada, that's treated under laws on statutory rape or child molestation. Although it does seem more nasty when there's a familial relationship, and in such cases "incest" may be an included offense, it's usually prosecuted largely without regard to the relationship. The state has the civil right and obligation to protect those who cannot protect themselves.

Incest is sexual relations between persons who are related "by blood," and does not depend on the ages of the participants. The "grounds" for it's prohibition are usually held to be that it favors the expression of recessive genes and produces "unhealthy children" who are a burden on society. In the context of this thread, most US states prohibit the civil marriage of first cousins or closer, and a few extend the ban to second cousins or even further.

Some might argue that the history of "close marriages" in European royalty in recent centuries shows that this ban is unneeded. Others might argue that the results of "inbreeding" of royalty proves that the ban is a good thing. That's probably not a subject to be argued here at this time.

John