I don't oppose nor even disagree, if they're lovers, I don't see why not marry.
If they're really willing to have kids, unadopted, okay to me. It's not as if the chance to have disabled kids is that much bigger for them than for non-related couples.
Moreover, if you say it is bad for the kids anyway, go tell that to 40+ women willing to have a kid too. The chance they'll have a disabled kid is very big indeed.