Well there's knocking on the door, and then there's blowing the door right off. In your comment you come across as judge, jury and executioner.

And to top it all, it seems that what you say is wrong. Apparently marriage to 2nd cousins has been allowed by the Catholic church since 1983, and you can even get a dispensation to allow you to marry your 1st cousin. So there's definitely no problem with 3rd cousins.

And the reason they banned it in the first place was to prevent accumulation of wealth by families, which was seen as a threat to the church. It had nothing to do with health reason.

(this information was obtained by a google search)