Good point.
Laws should serve only to protect individual liberty, not to engineer the exercise of that liberty. As a long-ago SC justice put it, "your right to swing your bat ends where my nose begins". Laws should not restrict your right to swing your bat-- should just prohibit you from bashing somebody else's nose with it. Nor should laws give special privileges to people who prefer swinging bats to swinging golf clubs.
If an adult brother and sister want to make whoopee together, I couldn't care less. Any offspring they may produce will be no more likely to be irreparably defective, than the offspring of the druggie welfare crowd, which serious Republican politicians have proposed bribing (with taxpayer funds) to "get married and stay married" -- as they continue to drink, do drugs, sleep around unprotected from pregnancy or diseases, skip boring nuisances like prenatal care, etc.