Can a falcon mate with a dove and have offspring?The answer to this will give you the heads up with what is natural and whats not.
Animals procreate for survival of their species and it seems that they have no choice.They seem to act on 'animal instincts'.Humans, it seems, can choose.
To me, 'sin' is only connected to the 'WHY' you have offspring with another person.If you are wanting to be with someone based on their race and think this is some cure for racism or racial prejudice then I could see that this would be a 'sin'.
Maybe we are at a place in our evolution where we are not capable of being fully aware of why we are with someone of a different race and so it would be best to leave it alone.Who can judge.
One thing I dont understand,though, is why people allways try and say that being 'equal' means we are all the same...just like saying that our freedom of chooice and 'group' belongs with the 'same' group and we are the 'same as' animals. If we are not entirely the same as the animal kingdom or the plant kingdom(in our basic function)then how are we different?Tell me the difference and let me know where the sin is involved.You have talked about animal kingdom and each species/groups' alignment and tried to make a comparisson with human beings and their alignment as though they are the same, but they are not if you study the reproductive possibilities.Even your Koran talks of the difference between man and animal.
