1:Humans as a species mate between the male and female, and this is the only natural (Test tubes & sperm banks and all of that are not methods given from birth, which is what I mean by natural, meaning abilities allowable at birth), anyone past the age of 10 probably knows this.

2:With Homosexuality, the possibility for a natural child birth is impossible, not the right equipment (Gender-Specific Organs) for such a thing.

3:In the cultures of the biblical era and beyond, creating offspring and the act of procreation for strictly that purpose was likely the normal cultural habit.

4:This notion is supported by several other cultural curiosities (From the perspective of the modern, western industrialized world, i.e., today, 2007) as seen in various ancient cultures, particularly, Homosexuality in Ancient Greece.
a. b. c. .
- So as explained above, it would seem that they did not view gender-preference as we did, but they held the act of being sexually awakened, or breaking virginity, as something of value and worth.
As for women?
d. A look at the typical household of Ancient Greece will give more insight into the views on Marriage and love:
e. And in regards to marriage specifically:

f. As you can see above, Marriage was much closely tied to producing children and political alliances (Although this was more prominent in Roman culture rather then greek). Very little is mentioned of Marriage for love.

- From the above analysis, it can be surmised that Marriage for Ancient Greeks (Who were also a prominent figure of Homosexual relations) was more of a business transaction then a rite of romantic love. Thus pure romantic attachments came from men-to-boy shared love. I could not find any records of Male-to-Male marriage, so I assume that practice did not exist or was illegal.

- Now then, for the second part: Homosexual love was not seen as a method of childbearing, so it was unnatural in a biological sense. In a cultural sense its clear to see that since a love between two men or two women would never produce offspring, that it was not of value and so it should be eliminated. In the ancient cultures of Greece, China, Roman Empire, etc., the lifespan of humanity was much lower then it was today, so there was a much greater emphasis on keeping the line of fore-bearers and the "House" of a Family alive, and this was done by child begating child begating child, ad infinitum.

- Now that we have established that, we can then perhaps imagine, that the original instructions of God were perhaps, indicative of the notion that Marriage and the act of sexual intercourse was best reserved for a man and a woman, as they would produce a child and the family would continue. It would likely be a hard 'sell' to allow for homosexual love when there was no obvious 'effect' that came of it (Sex between a man and a woman gave a child, thus it had an 'effect'), so God says "Marriage is to be between a man and a woman".
Take this original transcript, and attempt to convey the context and word-for-word meaning of what was said, combine that with the human element-- translating a word as something else because "Well, god OBVIOUSLY meant Travel and not Traverse" (An example of a word being changed), and add up many many years of small changes and language translations and add to that the bigotry of biblical text which does not seem 'correct' to a powerful group of men (Look no further the the Apocrypha Texts), and have that cycle continue for well over a few centuries, and you will end up with a translation of text which is far removed from the original source in literal meaning.

Of course, for those who would scream at me 'heretic' and 'blasphemer', keep in mind that even today, translators work on a 'meaning' and the 'intent' of what is said, and not the literal word-for-word message. This is why word-play jokes, puns, culture-specific references, etc., are often vastly different then the original source.

An example from the movie "Lost in Translation", which as you might tell by the title, deals with this very issue:
As you can see, the original meaning and the English produced were very different. The same meaning was carried across "Intensity and to the Right", but Passion may mean a whole something else to a Japanese than to an American, and while Intensity is a valid word to be used in place of Passion, Passion itself carries connotations not seen with the word 'intensity'.

So, to finally answer your question lurker, the non-emotional base reason, I feel that a lot of people do not consider Homosexual love the same as the love provided by God is thus simplified:
- Homosexuals do not have the right equipment for making babies.
- Babies are the primary reason for Marriage up until perhaps 100 years ago (Marriage out of love)
- God intended (This is my theory and not to be held as hard fact) that Men focus on producing children, and the bit about the homosexuality was probably that "Hey, homosexuality is fine, but once you get married, drop it, ok?", meaning that the priority of producing children and the sacrament of the act of sex should be reserved for people who with you share a deep emotional attachment with. One night stands and such are likely what he was trying to say was bad, and that while you can certainly have a deep emotional connection with a person of the same gender, that Marriage to that person was a symbol that they would not only be the producer of your child, but also your life-time partner and lover. THAT is what the meaning of marriage was original intended for, I hypothesize.
- and to conclude (The near-complete answer to this, covering ALL possible aspects could be a doctoral thesis in and of itself, so I will stop short) this expose on your question, lurker, I think that by a combination of Tradition, Childbearing/Biology coming first, and the over-time corruption of Church Members and Officials to fear and eliminate that which did not fall in with the teachings, is why today Homosexuality is viewed as wrong, and not of the same kind of love as the kind indicated by the Bible.

http://i14.photobucket.com/albums/a320/GeopAg/PDA.jpg

I realized that after typing out my post, (And a particular comment you said helped affirm it) that the central problem with the division between Same Sex Love/Marriage & Traditional Marriage/Love is that while any person in the modern world (I do not of course, refer to dictatorial or societies with very strict social standards) can apply for Civil Unions, there is the Symbolism that goes along with Marriage. Its the meaning behind the meaning, the mechanical devices behind the smoke and mirrors as it were, that by pledging to give themselves only to their appointed partner, they are telling the world (And with their partner obviously! =p ) that they are willing to be with that other person through the oft-repeated vows in any situation.

That symbolism and the ceremonies that go along with it, are a unique artifact of the days of "Rule by Religion" and the historic place of the Church as a body of power and legislation, and it signifies as I stated above, a commitment.

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Took the words right out of my mouth: It would also seem that those gay and lesbian couples are, indirectly, being denied both the responsibilities and privileges gained by Marriage, but also the SYMBOLISM, the act of saying "I will be with you no matter what", and it is a token of that couple's affection and love (And yes I am perfectly aware of the divorce rate, I will get to that soon), and by not being allows to marry, they are also being denied the chance to show to the world their love and devotion to the other, which is, Ironically, more powerful and perhaps (With the divorce rate as it is) more meaningful that they would drive to gain that which has oft been tossed aside by heterosexual couples.

Funny enough, I think that if Marriage WAS allows for homosexuals, it would bring at least in some ways, a reknewed sense of holiness to the act of marriage. Their struggle for equality in the eyes of the Church (Vicariously being the eyes of God) is also a shout-out that they want to follows the Church's rules and doctrines, and the church is caught in a sort of catch-22: If they allows gays to Marry, then in the church's eyes the act of Marriage will not be worth as much, but if they don't allow gays to marry then they will be denying doctrines of their faith, and similarly telling otherwise perfectly law (by which I mean the laws of God) abiding believers that despite that they go to church every Sunday and particpate as much as anyone else does, that they can't marry or find true (And not hollow-surface only) acceptance from the Church. This seems to be, contradictory and in some ways, insulting.

But the sad fact of the matter (And I speak often here in opinions and theories, little of which is or can even be made fact, just educated guesses and supposes) that In reality Marriage not worth as much because of the rampant divorces and materialism that is a fact of life today (Prenuptial Agreements, anyone?).

To address your second question: That is a very interesting question you pose and I find myself without something to respond with. As far I can think of, there IS no fine line between what is acceptable and what is not, and this... erosion of Ethical and moral lines (Which I also feel define themselves through the creation of strict and easily seen 'boundaries') is not only a question of what is natural and what is unnatural, but also a deeper question of how do we define morality and Ethics when the situations they handle more often then not have NO line at all... just a fine gradient, like the filter in photoshop.

You're a million times better than this gay guy that I knew in High School (didn't know he was gay until about a year after) who's married to a woman who's helping him "more than she knows"