Fact of the matter is that incest is way more common that you think and has been for time immemorial. There is no natural taboo.
Actually, there is a 'natural taboo', but it's directed towards anyone you spent much of your childhood in close proximity with. This typically applies to siblings and parents, but enough sufficiently close proximity can also apply to neighbors, friends, or close cousins. Anyone you've spent tons of your development years with.
Of course, since we're biological organisms, no built-in design works perfectly 100% of the time across a population. There's plenty of examples of sibling incest happening even if they spent their childhoods in constant contact, and plenty of 'childhood friend' tropes. On the other hand, there's also plenty of siblings separated at birth meeting up for the first time as adults and quickly getting amorous with each other. Sometimes even intentionally. And there's also the small chance that a good gene will be reinforced if inbreeding is allowed to happen on rare occasion. But allowing it to happen incessantly risks the Habsburgs.
What I'm trying to say is there are no absolutes on this topic. Not in any direction.