What about two adults with Tay-Sachs recessive gene? Should they be allowed to marry? Their chance of having a child with Tay-Sachs is likewise 25%? (Most modern Jews get tested before having children, but many, especially in the ultra-Orthodox community, do not)
Yeah, I certainly dodged most of this issue by saying brother/sister as opposed to father/daughter! With siblings, I think the risk is no greater than with any other relationship - in truth any marriage may have been coerced.
I think this is a logical fallacy. Everyone can choose to marry who they want, but I am not allowed to, because I happen to share parents with my beloved. Or, vice versa - everyone is allowed to marry a person of the opposite sex, including a gay person; there is no discrimination here. Your logic regarding polygamy is probably right, though.