The mere word incest triggers strong feelings of revulsion in most people. Therefore, any a priori labeling of a marriage as incestuous tends to preclude objective thought about the permissibility of the particular form of the marriage prohibition a tissue ("at issue", but I am dictating this and thought the AI error was funny). Such revulsion stems largely from the confusion of incest with sexual abuse of children. This confusion is not limited to the general public, but extends to the courts as well. The courts, then, are just as confused as the general public.
When criminal incest statutes are invoked, the overwhelming majority of prosecution's are for sexual intercourse between relatives within the statutory definition of incest, not for attempted marriages between such relatives. In a random sampling of 101 criminal Appellate Court decisions on cases alleging violation of criminal incest laws, all 101 cases involve prosecution's for sexual intercourse. More importantly, in 96 cases in which the ages of the incest participants were revealed, 94 involved in adult defendant in a minor child victim. Finally, 94 cases involved father daughter, father adoptive daughter, or step-father step-daughter sexual relationships.
The societal panic/paranoia about incest, however, stems from inbreeding which becomes irrelevant in father adoptive daughter, or step-father step-daughter sexual relationships. (Mother/son issues follow suit, of course.)
Inbreeding results in homozygosity,the state of possessing two identical forms of a particular gene, which can increase the chances of offspring being affected by recessive or deleterious traits. This generally leads to a decreased biological fitness of a population (called inbreeding depression), which is its ability to survive and reproduce. The avoidance of expression of such deleterious recessive alleles caused by inbreeding, via inbreeding avoidance mechanisms, is the main selective reason for outcrossing. Please note that homozygosity increases the chances of deleterious traits; it does NOT guarantee decreased biological fitness.
My point is that an "illegal" incestuous relationship is prohibited socially as well as legally yet does NOT necessarily require it to be a bad thing biologically. I would suggest that, should a PIV encounter arise (no pun intended), some manner of birth control is available to make homozygosity a moot point. Then, it's just a matter of keeping one's mouth shut... damn, there's a pun there, too. You get my point, though?