i agree with your second post (other than the last sentence) where you IMHO correctly point out (and extend jks' argument -- the (b) part below) that:
(a) various cultures do accomodate/permit or encourage marriage between cousins. i am not much moved by arguments that you may break-up the family if you end up getting divorced. i suspect such an argument is frivolous, but if not, i can offer counter-arguments.
(b) most of this sort of moral philosophy is a rationalization of "ick"iness. jks already pointed out that "ick"iness about homosexuality was expressed as a sort of moral judgement, but nobody dares do that any longer (at least in a left forum). that is not because it is politically incorrect, i hope, but because we realize there is no moral argument against it and (otoh) many arguments against opposing homosexuality. i disagree with your last sentence of your second post, where you suggest that such an approach to moral philosophy is fine with you. philosophy, unlike cultural mores, should be analytical. there is no grounds to continue to ground it instead in "ick"iness and such. as per jks' argument, it is imperative that we not categorize incest with "child abuse". the latter can be shown (given a simple framework that we may all agree on) to be wrong, whereas the former cannot be (IMHO).
but since i believe that moralizing of this sort should be in the framework of individual freedom and common good, i do not care what goes on in the minds of people. IMHO, to do so is to put moral philosophy in opposition (or confrontation) with biology.
--ravi