Good biblical reasons. It's not as clean cut on genetic grounds.
What incest will do is to bring out recessive genes. These genes can be either bad or good, depending upon the particular allele.
Not "or", but try instead "bad and good". I realize that with the technology avalible in the next few years we may be able to "weed out" some of the bad recessives, it still strikes me as a good idea to do our best to prevent incest/inbreeding in humans.
Remember the last bunch that went for "selective trait breeding"??? They walked really close to the inbreeding line in their attempt to create the "Master Race".
On the other end of the spectrum is the unrestricted and random inbreeding that has/does occured in isolated populations or in populations with little moral/sexual restraint.
That rarely brings out anything but bad.
The British Royal Family, prior to Princess Diana, makes a good case for the BAD traits coming out. She certainly improved the gene pool of that family.
The arabs have been inbreeding for millenia. Any questions?