Obvious point however, incest is not a sexuality. You don't hit puberty or whatever and think "gals don't do it for me, guys don't do it for me, but what I really want is a blood-relative", so you can simultaneously support gay rights on the basis of equality, and oppose legalisation of incest for all the myriad of reasons posted here. And no one has come up with a better reason than "but its a victimless crime". Which a) its not
B) Jakebert/Jackknife (can't remember which level-headed 'J' made this point) opens up a legal grey area for abuse to be easier to get away with.
C) Incest practitioners are not discriminated against in any other way except you can't get married. Fine by me. I will actually say right now I find it irresponsible, selfish and wrong. Which is an entirely non-controversial statement.
So in short: No net-advantage is gained from opening up policies around incest, however there are significant downsides. This is a basic way to measure the value of an idea, and obviously this means it is worthless at best.