Marriage prohibitions

No Right Turn argues for the adoption of his marriage equality bill, which would have New Zealand follow Iceland in allowing not just civil union but also gay marriage. My first preference is of course to get the state entirely out of the business of sanctifying unions, but if it's going to be in that business, I'm with NRT.

In his proposed bill, though, I found a surprising bit: Schedule 2, Forbidden Marriages. But it follows entirely from the current legislation. Here's the full current list of forbidden marriages:

1. A man may not marry his
(1) Grandmother:
(2) Grandfather's wife:
(3) Wife's grandmother:
(4) Father's sister:
(5) Mother's sister:
(6) Mother:
(7) Stepmother:
(8) Wife's mother:
(9) Daughter:
(10) Wife's daughter:
(11) Sons' wife:
(12) Sister:
(13) Son's daughter:
(14) Daughter's daughter
(15) Son's son's wife:
(16) Daughter's son's wife:
(17) Wife's son's daughter:
(18) Wife's daughter's daughter:
(19) Brother's daughter:
(20) Sister's daughter.

2 A woman may not marry her
(1) Grandfather:
(2) Grandmother's husband:
(3) Husband's grandfather:
(4) Father's brother:
(5) Mother's brother:
(6) Father:
(7) Stepfather:
(8) Husband's father:
(9) Son:
(10) Husband's son:
(11) Daughter's husband:
(12) Brother:
(13) Son's son:
(14) Daughter's son:
(15) Son's daughter's husband:
(16) Daughter's daughter's husband:
(17) Husband's son's son:
(18) Husband's daughter's son:
(19) Brother's son:
(20) Sister's son.

There's some clarification later on noting that wherever husband or wife is used, the prohibition extends also to civil union partners.

Now, some of these prohibitions will be in place because of heightened risk of foetal abnormalities when parents are close relations. But, we don't forbid marriage of those who are both carriers of the gene for other abnormalities: otherwise, we'd require carriers of the sickle-cell gene to check whether their partner were also a carrier. Moreover, none of the genetic arguments would continue to hold for same-sex unions.

And, a whole lot of the forbidden marriages seem entirely odd: you're forbidden from marrying your wife's mother. Now, I don't find that a binding constraint [nor do I now or have I ever found any of these to be binding constraints]. But it still seems really odd.

The best argument for some of these prohibitions would be the threat of undue power: a sicko raising a child to be his future wife. But the regs do seem rather overbroad for that. You're a single adult, your rich grandfather marries a nice young lady before dying a year later. You're forbidden from marrying her: she was your grandfather's wife.

I can imagine an argument that a bright line rule is efficient in that relatively few legitimate unions (by legitimate, I mean consenting adults) would be prevented while the costs of checking for undue power may be high; but surely an alternate rule of the form "forbidden if either party is under the age of 21" would achieve the same purpose. Is there any sense to the list of prohibitions beyond "some of these could be yucky"?

Shouldn't NRT be going a bit further in his recommended changes?