When opponents of same-sex marriage cry “Incest comes next!” they are relying on the deep-seated revulsion towards the notion of marrying a family member which can be found in nearly all of us to power this slippery slope argument. We have, however, known for some time that this revulsion has more to do with the participants having been raised together than their being genetically related. Brothers and sisters who were raised separately do not share this revulsion.
At any rate, feelings of revulsion alone cannot stand as an adequate legal argument against incestuous marriage nor against incestuous relationships generally. Neither can the legitimate claim that the offspring of an incestuous relationship is more likely to produce children with medical defects unless we are equally willing to deny marriage to people with Down Syndrome, Hemophilia, etc.
One argument I can think of that could be framed against incestuous marriage lies in the potential for abuse, particularly in the case of parent-child relationships (the older family member uses his/her influence to persuade the younger one). But with age of consent laws already in existence, I don’t consider this argument a strong one. My guess is that for better or for worse the incest taboo will eventually break down just as the homosexuality taboo is breaking down now. Check back in 20 years to see if my prediction is correct.