"That is true, but this is not a bad thing. It is because society advances. You could apply your argument to the 1800's in America; people would never have abolished slavery".
Why did people back then adopt the practice of slavery? It is simply because they didn't follow the religious teachings of their respective religions (every religion promotes peace amongst people). So therefore even though the major religions were established thousands of years earlier, people in America thought that it the practice of slave trade was moral. Think about it, and compare this to your argument concerning relative morality.
"Whatever the reason I don't really see an 'epidemic of bestiality' eventuating anytime soon!"
No one saw a epidemic of homosexuality 100 years ago but it is happening now. you say that there is nothing wrong with homosexuality, in spite the fact the the statistics for the amount of diseases associated with the practice speak for themselves. Lets not forget that homosexuality actually is against the human species reproducing, rather it moves it towards extinction.
You also say that incest is wrong as it creates abnormal babies etc. Well can't people use contraception? This goes against your argument supporting fornication. You assume that people won't use contraception when they practice incest, yet you assume that everyone uses contraception when practicing fornication.
"I think you'd have to be really insane or something to want to, but I don't think it'd be "wrong" in the true sense of the word".
Why would you think it would be insane for someone to practice beastality, yet there is nothing wrong with it? It sounds abit paradoxial to me. In the back of your mind you know that it goes against the nobility of humans yet because there doesn't APPEAR to be anything physically wrong with beastality, you arn't against the practice.