Legion:
1) Natural was defined, according to Webster's Dictionary, as that which is found in nature. That is the accepted definition of natural, and under that definition, homosexuality is indeed natural.
However, that is a completely different argument wrt the moral objections to homosexuality. The arguments should be kept completely separate because they are completely separate.
2) Human nature is demonstrably not different than that of other animals. We are mammals after all. Despite our intelligence, despite our "civilization," we are still floating on top of millions of years of evolution and animal instinct.
We are driven by three undeniably animalistic forces. Hunger, Sex, and the need for shelter. We are most certainly animals, despite our intellect.
3) Indeed, it is natural to have sex outside your species. As you have stated, it does indeed occur in nature. And I admit wholeheartedly that in earlier disputes wrt this, I got caught up with equating natural with good, which is not necessarily the case. Natural is completely separate from our moral objections of what is good.
It was once considered unnatural for interracial marriages to occur. It was once considered unnatural for women to do anything but bear children and live at home.
Now, my particular beliefs on sexuality and the species is that human sex should occur only with beings who possess the same, or greater, level of intellect that we possess. Why do I put it like that? Because if/when we ever make contact with alien races (yes I am being serious), there is no doubt in my mind that there will be intermingling, which would be "illegal" under the definition of "natural" sex that humans should only co-mingle with humans.
I also believe that sexual relations should only occur between those who are fully capable of making the decision, understanding the ramifications of their actions, and thus acting on that.
So under my open ended belief structure wrt to beastiality, underage sex, yadda yadda yadda, it would not be good for a woman to marry a pig. But if she wants to marry an intelligent pig from the planet Zoufrous
, then that's perfectly fine by me.
Incest should not occur because of the fact that it can produce horribly physically deformed and/or mentally disabled children. However, this is already expressed in the laws in this country as it is legal to marry your second cousin, or a more distant relation. Obviously this is because the genetic drift between the second cousin and greater is wide enough to bring the genetic problems brought up through incest to a more acceptable level.
I hope that's clarified the situation.