I am "tip-toeing" into this.....
It is true that some animals in nature engage in activity that would to us be perceived as "homosexual." But does this represent true homosexuality?
Take for instance the example of a certain type of frog. These frogs (I recall the observations but have forgotten the name of the frog) have been seen to engage in homosexual behaviour. Further research, however reveals the truth.
These frogs are stupid. Okay that isn't really supposed to be a joke; but these frogs have such simple reptilian brains they have no way to distinguish male frogs from females. They are essentially "pre-programmed" by evolution to a set of behaviour; when they come across another frog of their kind, they attempt to mate. If it is a female frog they succeed, but obviously if it's another male, they don't.
This cannot be said to be "homosexual behaviour," per se. It is in fact an evolutionary evolved instinct-driven behaviour that allows the specie of frog to survive.
Other situations may also drive "homosexual" behaviour, such as lack of members of the opposite sex. Is this true "homosexual" behaviour, or is it simply a means of venting built-up .... :mrgreen: .... "tensions?"
With animals I don't think we really can know, atleast all the time. Frogs are so primitive I think I'm on safe grounds claiming that it's a result of lower instinctual behaviour, not a matter of sentient choice. In higher animals the matter can become murky.
Earlier in history there were societies that did not disdain homosexuality as our present eurocentric "Christian" society(ies) does. During the Roman Empire, as well as the earlier Greek, it was not vilified in the way we do -- in fact in a lot of ways it was more the norm.
Yes, this is true. But remember what was considered more important back then was to procreate. Many children died before adulthood was reached and it was common for women to die in childbirth. Society could fail if there were no replacements and however one feels about homosexuality it is not a behaviour that causes replacements to happen.
It is said in the Bible that it would be better for a man to spread his seed in the belly of a "whore" than on a rock. Need I explain why? The "whore" can become pregnant.
That's why.
Now as far as the present day is concerned we no longer have a dire need to keep up "replacing" people -- in fact overpopulation seems more of a problem today.
Is homosexuality a sin?

Well, as has been pointed out, God apparantly thinks so. The Bible says it is. But then ... maybe it wouldn't today -- I mean there
is context and as I said our current problem is overpopulation. Just maybe today God is aware of this and would alter his ....."suggestions."
In any case it is any of my business to pass judgement, to "cast stones" -- since I am "without sin?"

Ha ha. No that is NOT the case:
I stand with Kevin on this. I don't care what people do in their private lives, so long as they respect me I respect them. And anyway, if God still regards it as a sin, that's HIS business. It is NOT my business. I do not know his will. But if he is omnipotent he will enforce his will.
....
And that, against me as well as other sinners, for we are all sinners.