First off, there is a potential qualitative difference between incest and homosexuality, and it has to do with imbalanced power relationships. The chances that an incestuous relationship embodies an unhealthy dominant/submissive relationship of some kind seems to me to be fairly high. Even if we’re not talking parent/child, the chance of an unhealthy sibling relationship is still sizable. The inherent risk of a psychologically unhealthy incestuous relationship merits separate legal treatment.
(No offense meant to healthy incestuous relationships. And there goes my chances of holding office.)
Second, if “it’s hard to provide a good distinction between consensual adult homosexuality and consensual adult incest,” then one ought also to mention that “it’s hard to provide a good distinction between consensual adult heterosexuality and consensual adult incest.” That is, after all, an equally valid logical result of the argument Volokh is making. Oddly, nobody seems to want to phrase it that way — it’s easier to lump incest and homosexuality together. Which is the same implication Santorum made.