Royals has married their cousins for hundreds of years. Shall we outlaw them doing it, too?
This has not been the norm in European royal families for a few generations now. Even when intermarriage among Europeans royalty was common, only a fairly low % of marriages actually involved first cousins. The Habsburgs were a bit of an exception.
The British royal family suffered terribly and royal houses of Europe because they could only marry people of royal blood. I remember Princess Diana being such a break with the past. Prince Philip a generation before was for the Greek royal house but Prince Charles did not marry a princess - a huge break but genetically a good thing. The Russian royal family intermarried with the British and others and had that poor little boy with hemophilia.
There is some strange history going on here. The hemophilia which radiated out among European royalty had nothing to do with consanguinity--it was caused by a single mutation which came from Queen Victoria, carried on one of her X chromosomes. (Consanguinity-related genetic disorders are to do with recessive genes, by contrast.) Queen Victoria's father was in his 50s when he conceived her, which may have been a factor behind his sperm containing said mutation. So if you wanted to use the law to reduce the risk of hemophilia, should we ban older men from fathering children?
I think cousin marriage is a grim tradition, but would be very leary of banning it on genetic health grounds as this could be a bit of a slippery slope (older fathers, older mothers, people with known genetic conditions etc.).
You could get away with banning it on the grounds that history suggests that widespread cousin marriage is incompatible with modern liberal democracy. Wherever cousin marriage is common, it is very hard to have functioning democratic government or a strong civil society (think of the state of countries like Pakistan, Afghanistan and much of the Middle East, and you will see what I mean).
All in all, though, I think it's better to educate people about the risks involved and wait for the custom to die out.