CathInfo.com

  It is a great Law that Cousin marriage is forbidden. In my Country we see the disastrous effects of widespread cousin marriage: 37 percent of birth defects are caused by cousins marriage. Also such marriages are more subject to elders interfering in personal matters of the husband and wife.
  Now experts in genetics and psychology are trying to discourage this custom but due it's religious roots it cannot be totally removed.  
 

It's disappointing to learn the SSPX grants dispensations so easily.

Thank for this.

Sure, my pleasure. If I may ask you, is this question more than theoretical? That is, are perhaps two closely related people you know considering such a thing?

Forgive me, but as I am not versed at all in canon law, I had look up "diriment impediment". This means that a marriage between first cousins would be "null and void" (not a marriage at all), correct?

If this is so, why is it possible to obtain a dispensation to marry without what would usually be considered forbidden degrees of relation? Or is it possible?

Yes, that is the correct understanding of a diriment impediment, but what degree of consanguinity nullifies a marriage is an open question, because not everything forbidden by canon law is void under divine law. But many other important considerations are taken into account when framing human law. Most traditional theologians and canonists held that at least first cousins would be incapable of validly contracting one.

If this were the case, then no marriage between first cousins could ever be dispensed. But since 1983, those who've accepted the new code at least have not retained that understanding. Apparently, according to the other poster, that includes the Society. But in any case, it is not common law, and still requires at least a dispensation from the hierarchy.

I can't think of any such Catholic marriages off the top of my head, but I can think of at least one marriage between two Orthodox first cousins that was dispensed by their hierarchy. Perhaps they have a different view of this.

Yeah, but they also have an incorrect understanding of remarriage after divorce, contrary to the words of Christ in the Gospel.

Does this canon law apply universally to faithful of all cultures, even if their native cultures place no prohibition on cousin marriages it one were to marry a cousin on the mother's side?

No, it doesn't, but natural law or divine law would. But like I said, it is still an open question what level of relationship by blood is actually rendered void.

While I think such a marriage between an adopted son and his foster parent's daughter would be absolutely forbidden, the question is interesting - when the Church places these prohibitions on marriage does it do so based on biological or official relationship?

There would be other grave and natural reasons to think such a relationship should be rightly forbidden, but it is the biological relationship that the law takes into account. That's what consanguinity or "blood relationship" means. As spouse of Jesus notes, we can see the Church has certainly been prescient in this matter in light of modern genetics and psychology.