Frankly I find the "because it's wierd" argument that some people here have landed on extremely bad. That has been exactly the reason behind sodomy laws for as long as they've existed, and we know those are bad. Luckily there is a decent argument against incest, because Prager is showing his ignorance of the topic.

Incest between siblings of similar age is vanishingly rare, but probably shouldn't be illegal because there's no real reason for it to be (and if you make it illegal, as in Germany, you create wierd edge cases where siblings who were separated at birth might hook up as adults and suddenly realize that their relationship is illegal despite that not making sense). Prager pretends that this is the only kind of incest that exists.

The real problem with incest is parents abusing their children and older siblings abusing younger ones. The power dynamic at play there makes it impossible to give consent, and highly questionable even if both parties are adults.