List your reasons as to why siblings should not be allowed to marry.
By humanistic/secular standards, I don't have an argument (I'm sure others do and will chime in....)
Personally, I think it is revolting and wrong.
But I also feel that way about homosexuals.
I don't hate anyone, however - I feel nothing but love and compassion for them - I can feel that way and still not agree with their lifestyle however, contrary to popular belief.
Personal revulsion is not a legal argument.
Correct.
The 'legal' argument, such as it is, against incestuous marriage hinges on the genetic consequences argument.
1. That separates it completely from any same sex marriage arguments and therefore demolishes any attempted analogies between the two.
2. The genetic argument itself is quite weak for many reasons
a. it is discriminatory because unrelated couples with transmissible genetic defects aren't banned from marriage
b. Incest marriages between couple who can't reproduce would logically be exempted from the ban. They aren't.
c. Some anti-incest laws involve non-biologically related siblings, mothers, fathers, sons, daughters, thus ignoring the genetic argument altogether.