If you study something
more of psychology, you will discover it's a very discussed matter also today (Westermarck effect, inbreeding avoidance, etc.) and that the Freud's research on incest (I guess you're referring to it) was a biased research.
Freud was just very famous, it doesn't mean he was right on everything; for example Einsten was completely wrong on quantum physics!
And if you think that's because of our social constructs....if you study genetics you will discover that animals (like we are) tend when possible to avoid incest because a mixed gene pool is stronger (that's actually the point of heterosexual reproduction) and it's statistically proved even in not social animals (they just use other means to avoid it, sometimes not just psychologically but even biologically!). Animals (statistically) just fuck relatives when they can't fuck anyone else (better something than nothing).
Also "against nature" is a funny word because dogs try to fuck everything it moves, seals rape penguins and dolphins masturbate with dead fishes! Humans are just sick fucks like everyone else in the animal kingdom XD
BTW I have a steady relationship with my GF (and she's not my first one), some friends, and a job I kinda like
So neither incest nor bestiality are normal (statistically are both considered disgusting) but, since these are just pixels and I'm not actually traumatizing a girl for life, if you like it I don't think there is anything wrong with it