Well sure, but genetic dangers don't always happen when families intermarry. Sometimes the kids come out just fine. I mean, we can't tell people who can have kids and who can't. If we do that, should we also discourage or illegalise people who have mental disorders, physical disorders, and cancers from reproducing? It could be passed onto children... If this doesn't suit the discussion, I'll make another thread on it.
I have heard it argued that a person with a genetic mutation should have the right to pro-create because there is not a person with which she or he can pro-create without passing on the gene, and telling people with genetic mutations that they cannot pro-create would open up a big can of worms.
On the other hand, people desiring incestuous relations could always pro-create with someone outside of their close family. That option is not available to someone with the mutation.
The contract system which could encompass all family structures, not just conjugal or romantic relationships, could benefit families -- for instance, a brother and sister living together, or an adult child caring for a parent, but that doesn't mean incestuous pro-creating would become legal.
The system would also be able to benefit polygamous families, polyamorous families, co-habitating couples who may not be conjugal partners, extended families, friends and/or family living together who care for each other and are financially independent, etc.
In other words, monogamous, romantic partnerships would not be given legal recognition over any other family structure.
