The cynical part in me would say that as far as people go, they need to be "scared straight" by something unimaginable but visual nonetheles. Esoteric discourse about why that's "bad" has a the same effect as yelling in the howling wind.Some religions forbid sex with animals because it was supposed to produce monsters. They also condemn wanking.
Religions rest upon the ability of the "crowd" to replicate itself and pass the religion onto the younger generation. Should you chose to get it on with members outside your species, not only you're setting a "bad example" to others, you're also not helping with the propagation.