Satya, I won't argue which definition of NATURE is better - because you seem to have a hang-up with the word UNNATURAL.
I have a hang up with semantics.
If the word 'nature' is used in a particular way, for example, as you propose, in referring to what occurs in nature, then I will agree that homosexual activity is natural.
However, I do not think that this is a good use of the word natural, because cannot function. Eventually, the use of the word nature, which you propose ends up to one of two conclusions:
1. Everything whatsoever must be called natural, even things like men deliberately dumping radioactive material on civilian populations, to divert a country's resources to medical aid. (This could be called natural, according to your definition, because it occurs in nature).
2. OR, a sharp, false dichotomy must be drawn between human life/activity and all other life or activity on earth, so that most human activity, like building houses or farming can be called natural.
Ultimately, if one adopts your use of the word nature or natural, then either absolutely everything must be called natural, or absolutely every human activity must be called un-natural.
I agree, the definition of nature can be drawn out to the point that it becomes virtually meaningless.
If however, one admits that humans are capable of some acts which are contradictory of their nature, then homosexuality must be un-natural.
Oh, so now you are arguing that homosexuality is contrary to human nature. Well then let's start it up again.
Correction. If people accept
your definition of human nature as the correct definition, then
you can argue it is unnatural.
What if the definition of natural is "that which occurs in human nature"? Then homosexuality is very natural because it occurs in approximately 5% of the human species and in virtually every human culture even where it is despised.
What if the definition of natural is "that which is in accordance with human evolution"? Then homosexuality is very natural because homosexual sex has been shown in social animals such as primates to act not only as a means of reproduction, but as a social bonding agent which improves the overall fitness of the population. (that one doesn't really need to be changed and you are welcome to address it rather than ignore it)
Both of those definitions of "natural" are just as applicable as your argument that natural is that which doesn't contradict human nature.
But I can even form an alternative interpretation from your definition. What if the definition of natural is that which serves human nature? Then homosexuality is natural because it provides the practitioners with the opportunity to form romantic/sexual relationships that they would not otherwise be able to form due to their biological predispositions. (didn't have to change that one too much)
As far as "intellectual choice", what purpose is there in not choosing to do something that is prevalent in human nature, is supported by human evolution, and serves human nature?
By my definitions, I can make a very strong argument that homosexuality is natural. Do you care to argue why
your definitions of natural and human nature are more correct than my definitions?