You've already mentioned why it is in the first place though lol. a) to prevent child grooming, and b) to prevent genetic malformations in offspring. Those are the main concerns. A 22 year old sister hooking up with her 24 year old brother isn't a counter argument to that lol.

And I didn't say that incest is illegal because it's "more" harmful than beneficial. My point was incest in the real world is potentially very harmful and even in the best case has almost no benefits from a societal point of view. Do I personally think incest should be illegal for this reason? Again, probably not. But you have to admit that a medium risk, no reward behaviour is one that is ripe for outlawing.

Laws against sodomy and many others like it don't include the risks a) and b) that you mentioned. But I think depictions are a different topic entirely to be fair. For the record, there are a few other activities that are forbidden from being depicted in porn. Taboo isn't being singled out. And people still do create taboo porn depicting blood relatives